Modern replicas don't use historical steel for the blade so why shouldn't guards and pommels be stainless?

Many mass produced lines do in fact use stainless fittings.

Off the top of my head, one big drawback of stainless steel is also its main advantage, namely that it doesn't oxidize like regular steel - it doesn't take well to etching, doesn't develop the same kind of attractive patina over time, etc.

Stainless is very difficult to forge compared with mild steel

If the modern steel being used for the blade is stainless, sure, why not. It's already been mentioned that it's harder to work, won't take a patina or true etch, and it is less forgiving in the long run if you're going to use it as a sparing weapon. However, if the steel used for the blade is "modern" but relatively analogous to historic steel (not stainless or some other non-natural alloy or an alloy not seen during the period of the sword represented), I'd want the fittings to use a steel analogous to the historic steel as well; or bronze. While I do have a Windlass that has a stainless blade and very historic powder coating on the pot-metal fittings, it is the only such sword I have and keep it as a reminder that historically truer is better. At least for me.

Hanwei uses a lot of stainless

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Stainless steel might be better regarded as corrosion resistant

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The result in these images are from applying a solution of hydrochloric acid, selenious acid and copper sulfate

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Ranging from mottled gray to glossy black, the "rusty" look in the first photo is the copper sulfate effect.

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This particular solution is from Caswell Plating and labeled (appropriately) stainless steel blackener.

https://www.caswellplating.com/stainless-steel-blackener-8-fl-oz.html
https://emeraldcoatings.com/wp-content/uploads/2018/02/stainless-steel-blackener.pdf

I have seen applications reaching near jet black glossy but I was going for more of a distressed mottled finish.

Cheers
GC

The general consensus on stainless sword blades is that it's hard to get the heat treatment such that the blade isn't brittle at sword lengths and with edge thin-ness. For solid hilt components that are not long and thin, it can be fine. Albion's Maestro line uses them as do some other makers.

:D With rostfrei being...........?