How advanced are the AI editor's priority's ?

I've been making my own AI for my maps. Before trying editing them .ai files manually I fiddled around with the AI editor that comes with FT's WE. So far I think I created a decent AI (as decent as it gets with the editor anyway), it builds all the buildings needed, units, hero's, expansions (conditions to keep several things incheck) and it uses several attack waves.

But I was wondering how far one can go with the conditions.

I tried to make the elves AI build up to 6 archers when it doesn't have a tree of eternity, but limit the building to 3 once the tree upgrade has finished.
This resulted in the AI stopping all further actions once the tree upgraded. I think because it wasn't able to build some units, while it needs to get everything top2bottom when processing the build priorities.

So, when I want this behaviour, should I start looking at JASS ?

Yes, it is much easier to make complex AIs in JASS.
I suggest you have a look at AMAI. The common.ai of AMAI provides a lot of useful functionality for advanced AIs.

But don't get me wrong. It is quite possible to do the behaviour you described in the AI editor but the more in this direction you do the more complex it will become to do that with the AI editor.