You can selectively disable most things in Xtra mod. (I also disliked a lot of the stuff in the mod and removed it.)
So why should I need a mod where I dislike and turn off most of things that mod introduce? :)
About the AI: Do you have some evidence for that? Because far as I understand Xtra mod supposedly improves the AI.
No, I haven't had much time to witness it personally, just because I uninstalled XD earlier :) So it is more a question than a statement from my part: how does AI manage in this mod (mods). I just base on my previous experience with big complex 4X games that have mods. Modding AI in such games is a most difficult task in modding as far as I can guess (at least modding it in better way :P).
And IMO original game has issues with too low quantity - at least the research "tree" is too scarce.
Yes, I tend to agree with you here. Although this feeling of 'scarcity' of research in FELH seems to be more psychological
(perceptive) than having the real impact on gameplay. As for me, I always play on huge maps, with abundant resources, plenty of quests, many of monsters and random events - but leaving Research pace and Magic power on Normal. And I can't remember any game where I'd seen that whether I or AI have no more techs to research and just loitering about knowing not what to do :)
The same could be said about troops/units equipment (derivative from Research though) - seeming rather modest if not scarce in quantity and variety, in fact I think there's just the exact quantity and variety of regular (researchable) equipment in FELH to keep things balanced and not overblown. Maybe, of course, it's just me :)