Praets are powerful until Gunpowder? Have you forgtten about Maces and Knights?
I like the Celts. A lot.
I think their trait combo mixes powerfully and with mysticism as a starting tech their traits can be levied into producing a long lasting synchronicity of tech discoveries, significant GP's and Wonders. They are equally adept in taking the warmongering route or teching into the more peaceful building pathways.
I look at the UB/UU issues from this perspective: I am not a wall builder. I do not see where the UB takes anything away from Celts. One can choose to not build Duns and no harm is incurred. One is still free construct barracks and stables and grab all the normal promotions. Adding another promo is hardly a bad thing, especially when it carries over with uprgrading. And since I don't find any of the UB's to be game breaking, unless they are variants on buildings I normally go for, they make little difference in most games I played.
The Gallic Warrior is limited in scope and a regular Swordsman would be a better attacking unit, no doubt. But, I like the variety that not all UU's are THE dominant unit of their respective Civ. It creates game dynamics that get the player looking outside their normal scope of vision to acheive success. Instead of building an entire game strategy around a single unit and using it to enforce a theme, the Celts deeply reward a type of play that matches up long-term trait synergies with long-term strategies.
First go through with the Celts I got pummeled & humbled. I found out that I needed to become a better player, one that did not have to rely solely on tactics such as CS Slingshots or brute force conquesting, in order to maximize the Celtic experience.