How is it some AI's cannot use all units?
Well, first code has to be written to use the unit. If the AI player does not understand the function of a unit, it cannot use it. The brain must be developed.
This will actually be something Unit Set creators have to be aware of - that if they make a brand new unit - the AI will not understand it as a concept. A brand new unit is very different than just "making the tank faster/tougher".
The only AI currently in the mix that is capable of understanding something new would be the Enhanced AI. But even still you have to explain it in a script to him so that he understands what context he can use it in (define Rules of Engagement, is it good at exploring? do it reduce drain? etc.). It is all very involved actually. and the results are not always what you hope for.
Now the exciting thing is that the three existing AI's can be changed to understand more concepts. I would think the "Classic AI" would be pretty hard to modify, as there is some legacy with the way it is setup. If you wanted to add the concept of a Satellite and what to do with it, it could be done.
The "Expanding AI" is very minimalist in getting its job done - and it is quite effective imho. It can kick your ass if you make a wrong move. But I have kept it minimalist for a reason so that hopefully others can come in and add to it. It is not a very complex set of code for that reason.
AIs are very challenging. and it is hard to do and quite time consuming. If we have members of the community that are up to the challenge (and I know we do), we should all cheer them on.