The ones in FC were basically just uniformed-colored flight suits, like made out of the stuff real ones are.
Which was fine for that movie, as it kinda gave the impression they were just made for protection in the jet, g-forces and all, but you probably wouldn't want that as a "custom, one-of-a-kind X-Men-made X-Men uniform" overall. They looked cool, but honestly the new ones do too. Wish they weren't all identical, but overall it's still by far the closest to like "classic, 80s Claremont X-Men" we've got so far. Hell, it's probably the closest to the comics you'd practically want to get anyway, the movies are a different beast. This look is sort of "MCU sweet spot" zone, reflecting the history but revamped & made more amenable to be used in live-action without looking laughable.
Then again, I was more or less fine with the leather jumpsuit original-trilogy Singer-era stuff, so what do I know?
But yeah, the black leather criticisms felt legit. Here it's kinda like...people will complain either way, this is the closest we've had to the comics so far, I'm good.