I think that any time you base a fantasy stereotype on a real-world culture, you practically open the door to those who are looking for a reason to be outraged by something.
It doesn't help much that so much of the fantasy and sci-fi hobby, for better or worse, is a product of the golden age of pulp literature, which gleefully saddled itself with all sorts of "unfortunate implications" drawn from the anxieties and fears of the time, cultural stereotypes, and pseudosciences of the time.
I'm not even sure how many pulp writers genuinely believed in anything they put in print: based on some of their outside writing on how and why they wrote the way they did, it seems that pulp writers - paid a few cents per sentence to deliver a viable story on a short deadline during the Depression where money was hard to come by, the pulp writers seemed to rely heavily on shortcuts - such as plots that are written as westerns but can easily be rewritten as sci-fi adventures or as the adventures of a medieval warrior against savage Indi- er, I mean orcs, and so on, or, unfortunately, grabbing simple stereotypes and playing them as a sort of shorthand that the audience would recognize without needing any complicated exposition or cleverer writing to get to, when the audience doesn't really care WHY the orcs are on the warpath, they just want to see their favorite pulp hero get into some sort of suspenseful cliff-hanger, and then find an exciting way out of it, with some dangerous-sounding monsters, eerie locations, deadly pitfalls, and so on along the way to help give it all the right atmosphere.
Not to say it's good and right, nor that there weren't people who also wanted to believe in the stereotypes and enjoyed those for whatever they were worth, but rather, I mean that for better or worse, the pulp era is the foundation of our fantasy tropes, and those tropes have a way of coming back to haunt us if we aren't careful.
And in the case of D&D's hobgoblins, we've got a Lawful Evil, expansionist horde of people united under a diabolical purpose and organization, who are out to conquer the world... it doesn't seem bad at face value, until one remembers the old pulp-era Yellow Peril trope, and then realizes we're talking about a horde of warlike, conquering "monsters" with green, orange, or yellow skin, dressed in Samurai and Mongol outfits, for things to start getting more uncomfortable (especially since in the 1980s, to avoid some of the Yellow Peril criticisms, the evil hordes of Ming the Merciless and similar clones of clones of products of the "Yellow Peril" cliche were simply recolored with green skin, with nothing else really changing! The D&D hobgoblins, whether by accident or purpose, seem to have been given a makeover in the '80s that simply gave them a "Yellow Peril" theme with a different colour....)
I doubt that any of the fans of the Samurai armored hobgoblins are thinking of anything like that, of course - rather, the Samurai armor is a part of the hobgoblin tradition thanks to older illustrations, and, darn it, the Samurai armor and weapons look really cool together: they were made by artists and craftsmen with access to a rather sophisticated industrial capability (seriously, the manufacturing processes for these weapons and armor are amazing), with an eye toward grace, harmony, and beauty that is difficult for an artist to simply ape as a similar-but-distinct clone that looks like a product of a different, original culture. For that matter, Ming the Merciless and Fu-Manchu - products of the same stereotype - also look darned cool!
So anyway, I don't think anyone today means anything by the Samurai armor thing, and I don't even see it as a necessarily bad thing to start with a stereotype like the "Yellow Peril", and then play with the expectations a bit in ways that add a little nuance and comlexity to the stereotype.
There's a lot of ways of doing that, but one might be to combine pleasing and cool elements of the Samurai armor with creatures that don't fit one of those old stereotypes (like dwarves or other good demihumans? Skeletal warriors? Ordinary human soldiers with a variety of motives beyond conquest and evil?) You're basically rewriting the rules of the hobgoblins to something that distinctly different from the stereotype.
Or, design hobgoblin armor that borrows only loosely from Samurai or Mongol inspiration, but ends up looking like a distinct and alien culture with its own unique flavor quite different from anything suggestive of the stereotype. (Basically, it's no longer Samurai armor....)
Alternatively, you can design the world around the idea: why are the lawful-evil expansionist horde in Samurai outfits? Because everyone else is wearing he same outfits, in a world inspired by a carefully researched feudal Japan! (Which is a bit difficult, if the othe fantasy creaures in the product line are inspired by western mythology.....)
And then, there's the interesting possibility of drawing your Samurai monsters not from western fairy tales by way of pulp literature, but something a little different: the hobgoblin is normally based roughly on a reinterpretation of the European Unseelie Court folklore and fairy tales, with some pulp cliches tacked on, but what if you looked instead to Japan's Yokai - the Japanese equivalent of fairies and goblins - for inspiration, to create a sort of monster army inspired by the same culture that inspired the Samurai armor and weapons? For a few of many kinds of Yokai spirits you might draw from for inspiration, see:
- Schichinin Misaki (an interesting Samurai-themed Yokai goblin with elements of ghoul, zombie, or vampire mixed in, as a spirit of vengeance and pestilence found near the water where it died - those who kill it are cursed to become a Schichinin Misaki themselves)
- Ashura (a warlike "hobgoblin" from the Bhuddist tradition with multiple arms who are ruled by violent passions)
- Yasha (warrior nature spirits tasked with guarding the treasures of the Earth - they aren't exactly evil, but they do have a taste for human flesh, and are fierce warriors, regarded in Japanese traditions as being similar to Oni devils)
As an aside, strictly speaking, it seems from what I've been reading that most of the soldiers in a "Samurai" themed monster army kit wouldn't actually be Samurai in the sort of armor with the sort of weapons we might picture - such high-ranking Samurai warlords would, I take it, actually have been more like a "command sprue" leading an army of slave-conscripts or hired soldiers - Ashigaru - dressed in simpler armor and conical helmets, generally carrying pole-arms, guns, or - if they are especially well-armed - some basic swords of a lower quality than the elaborate status-symbols of a Samurai such as a Katana.
Ashigaru - Rank-and-File footmen serving Samurai warlords....
One's mileage may vary, of course, on how well it would scratch the right itch for a "Samurai hobgoblin" army kit, but I can't help thinking a boxed set of demonic corpse-like Ashigaru reanimated by their violent lust for war to serve a powerful Schichinin Misaki-inspired hobgoblin warlord in fearsome Samurai armor, driven to haunt the earth from beyond the grave because of an unquenchable quest for vengeance, sounds eerie, cool, undeniably on-theme for the Japanese armor and weapons, and I think (hope) it's quite a bit fresher and more interesting than the standard-isssue pulp-derived hobgoblins as depicted in D&D, without being completely dissimilar to the basic rule-of-cool Samurai hobgoblin concept!
Schichinin Misaki: Japanese Goblin Samurai with some vampiric qualities...
I get the impression it's absolutely in the right spirit of things to add hobgoblin-like fangs, horns, and claws to the template as you wish, as those would be quite in line with depictions of Oni (the Japanese word for "hobs", or devils....)
Regarding the question of why one couldn't combine some Samurai and Mongol influences, I don't see any reason why not, aside from the pan-Asian conspiracy angle from the Yellow Peril stereotype. The Samurai were around for a long time, but really saw their first big success and rise to power when they helped repel a Mongol invasion, which, though a superior fighting force when the invasion of Japan was launched, was decimated by the original Kamikazi - a divine wind that swamped the Mongol navy, drowning many of the Mongol warriors, leaving the Samurai to finish the job of saving the country from the rest of the invasion! So, the two cultures did meet in warfare, and one might imagine a fantasy version of the story in which a Japanese Yokai hobgoblin army might have inherited a mix of the armor and weapons of both cultures. Not strictly necessary to blend them, but I can see a backstory where it works!
By the way, thanks for this discussion, this was a fun subject to research! Though familiar with it as a pulp literature fan, I didn't know a lot about the Yellow Peril cliche or its evolution, or about D&D and Warhammer hobgoblins, or Yokai (beyond the occasional Japanese horror or fantasy movie), or Samurai and Ashigaru and their amor and weapons, history, and armies before now!
Bear in mind that I'm not an expert on any of these subjects, and the subtleties of Bhuddist, Hindu and Japanese pagan philosophy in particular are going to be lost on me (I could tell they were a running theme through a whole lot of what I was reading, especially the Yokai, but I can't claim to understand anything but the shallowest take on them); I included links wherever possible to more detailed articles for anyone interested in making any corrections, or building on the Yokai hobgoblin concept.