Firstly, access to substitute isn't special. Everything that can learn TMs can learn Substitute.
Furfrou does have a good ability, but it doesn't have very good tying. It has one immunity and no resistances. Steel types will wreck a Toxic stall Furfrou. If Furfrou meets something it can't stall out, it has to switch, losing any Cotton Guard boosts.
It also has no reliable recovery method outside of Rest, which means you either run a RestTalk set, which is viable, or spend 2 turns as a sitting duck. Even if you run a RestTalk set, your options of other moves is really limited. You either forfeit Cotton Guard or Toxic, otherwise you're totally susceptible to Taunt.
Basically, there are superior walls, not least Regenerator Audino, which has a much better support move pool. Carbink outclasses Furfrou because of its ability to set up screens/ rocks, and Dragalge at least can't be poisoned and has better defences. Furfrou essentially has a lot of limitations as a wall, meaning it's not really anyone's first choice for stalling purposes even in NU.
