Is the Artificer Armorer's Extra Attack feature limited to the Arcane Armor's simple weapons or any weapon the Armorer is proficient in?
Extra Attack: Starting at 5th level, you can attack twice, rather than once, whenever you take the Attack action on your turn. (Xanathar's guide)
RAW: seem to say it functions like any ability that grants Extra Attack (Fighter, Monk, Ranger, Warlock's (Thirsting Blade).
RAI: seem to be only for the weapon the Arcane Armor has.
So which is it?
There is nothing in the text of extra attack that specifies you need to use a weapon you have proficiency in from your class. There is nothing in the text that says you have to have proficiency at all. You could attack twice with a greatsword if you wanted to, even if you lack the proficiency for it and have a low strength score.
The Armorer Artificer's 5th level Extra Attack feature states, in full:
You can attack twice, rather than once, whenever you take the Attack action on your turn.
If there were any restrictions in terms of what weapons you could use they would be included. An example of a feature that does include such restrictions is the Thirsting Blade Eldritch Invocation which states (emphasis mine):
You can attack with your pact weapon twice, instead of once, whenever you take the Attack action on your turn.
The mechanics of the Armorer's Extra Attack feature are contained within the feature itself, there aren't any hidden rules or restrictions. Well... except one from multiclassing:
If you gain the Extra Attack class feature from more than one class, the features don't add together. You can't make more than two attacks with this feature unless it says you do (as the fighter's version of Extra Attack does). Similarly, the warlock's eldritch invocation Thirsting Blade doesn't give you additional attacks if you also have Extra Attack.
