AI gunners don't use additional ammo types unless you force them to, and AI engineers don't know how to buff (and I honestly can't think of a way that Muse could make them buff without making them too good at it).
With that in mind, I think that all AI should be engineers with mallet/spanner/extinguisher.
From my experience.. Three engineer AI would be extremely limiting.. As you are basically either in tank mode or in no repair mode .. AI engineers will shoot in "as you were" if there is nothing to fix.. but if your under constant attack then they will not be shooting.. The AI gunner lets you have someone shooting preferably and two repairing.. (It might work for long range builds where you try to just tank while your ally gets them off you and you pull away.. but not in close range).
What I do want to see is that the game make sure it never puts a ship in to a 2 gunner one engi situation... Which has happened to me quite a few times... and usually results in you being an easy kill.
Also AI gunners should have a wrench not a spanner.. (I am pretty sure they have a spanner?)..
Also doesn't the AI reload guns if they have a non-normal ammo in them? They always seem to do this to me..
I think if they kept it limited to just being able to set the AI to only repair the parts you want it to.. You could make them more consistent/useful with out making them a lot smarter.. (IE like Zill says they can't jump but they also can't use shortcuts in general (like hitting hull of junker from the front, center pyra engine from below)..
They also, I think, rebuild slower then a human who clicks in stead of holds down the button.. They can't keep shooting if a part they are told to fix needs to be fixed.. They have to fix it right away (so mallet hit on fresh dmg often)...
All these little flaws would still let humans be better.. But I agree somewhat that we don't really want the AI to be so good we don't want to play with a new human player (We would still prefer people who know what they are doing even with this change.. just less so for people who don't know ).
If they did add this they would need to add a new ai command that was like "Custom" so the usually default is there.. but they would also have to implement some way for you to know what AI your getting.. Am I getting the AI I scripted to be a gunner? the main engi? the balloon engi? The whatever.. (Easily done with a priority list.. so if two humans use ai A if 1 human ai A and B and if three all three)..
I have thought about this before but usually come to the conclusion like redria's we don't want AI that is to good and so maybe the reason why features like this haven't been done..
I wouldn't mind seeing this in adventure mode(maybe co-op to?).. Since I think that will be playable solo/single player yes? (Not real on top of that.. clearly haha)... No balance issues necessarily in that if that is the case.