Basically, without NWNX:
-Ability score bonuses are removed then reapplied after resting; can cause issues for caster classes if said ability score bonuses are linked to casting (i.e. int/cha/wis). In other words, you can lose some prepared spells or uses per day.
-Bonuses are considered magical, so they count towards the engine's limitation on how much you can stack (i.e. you cannot have, say, +50 Intellect from buffs or items).
With NWNX and the appropriate stuff:
-Ability score bonuses are identical to racial bonuses (i.e. they stay around no matter what)
-Bonuses are not counted towards the maximum you can stack.
In other words, if you're playing a casting class that uses a casting attribute that isn't affected by the template, or not playing a spellcasting class at all, and you don't plan on stacking lots of magical bonuses for the same thing the template grants bonuses for, then you don't really need to mess with NWNX.
But to get the most out of templates, you do need NWNX, which lets you bypass engine limitations.
