I started thinking about all purpose and/or all powerful allies and the ranks are thin. There are only 13 allies whose willpower, attack, and defense are all two or greater. Interestingly, lore does very well in this regard, as six are neutral and five are lore, with one each for leadership and tactics. Of the 13, six are two across the board for willpower, attack, and defense and could be considered all purpose, but not all powerful. Four of those who remain are powerful, but have serious limits (such as being discarded at the end of the round).
Taking it a step further, to these 13, I added allies who were 3 or greater for at least one stat. Only two additional allies have a willpower of 3 or greater (Denethor, Galadriel). Four more have a defense of 3 or greater (Beorn, Defender of Rammas, Warden of Helms Deep, Winged Guardian). Thirteen additional allies have an attack of 3 or greater (I won't list them here).
These leaves us with 33 allies with all purpose to powerful stats, but few that could could considered all powerful. All powerful can be defined many ways, but I settled on stats of 3 or more in two categories without a discard at the end of the round effect. The likes of Treebeard, Gildor Inglorion and Beorn are rare indeed. None of this takes into consideration ally abilities, which adds a whole other dimension to their power. I'll leave that discussion for another day. Nonetheless, I was surprised to see the number so low and realize this speaks to the game's balance.