It's more mechanics that inject themselves in the common area bogs the game down. Weapon variants. Skill variants. New spells. Feats that gate off common actions.
Classes and subclasses that offer new options and ways to play? Well if the people want it...it's all positive.
I think those things are not all the same.
Weapon variants are an overall great thing, it adds to the warrior's toolset. If you are worried (very justifiably) it dilutes weapon specific feats, just use weapon groups as in the SRD, or even without restrictions.
New spells are overall a good thing, a lot of good effects and tricks can be found there. Many caveats intrinsic to magic though - like at which level a given spell should be available without trivializing exploration. I would have preferred a stricter ruling about not adding spells to spell lists, but swapping them. It's weakly suggested as a possibility in a few 3e splats, but not vehemently enough.
Skill variants, I don't know what you mean.
"Feats that gate off common actions" is debatable. Are you referring to maneuver feats as an example? Those are ok if limited to 1. The issue is chains not designed as an actual chain but as mere improvements that should be automatic (in this 4e was correct, I say this as a non-fan). Probably the best idea is also also add feats that give you alternative routes, like the Dirty Fighter pathfinder feat. Ironic, because Pathfinder split up maneuver feats which was a massive mistake.
What else is gated off? Stuff like Spring Attack? Miself, I find 5e movement too "free" (I am aware I am in a strict minority) but I think it works only if many monsters and possibily PCs are played in a suboptimal, especially at low levels.