Well, this post is commenting on two separate things:
1. The fact that the solar was left roughly the same in power as in 3.0 while the pit fiend and balor were bumped up in power substantially.
2. The fact that the solar, a paragon of goodness, virtue, wisdom, majesty, and fortitude, has lower Int, Wis, and Cha scores than the pit fiend, an otherwise less powerful creature overall.
#1 is easy to answer. The 3.0 solar was already radically overpowered for its CR (a number of reputable posters and game designers put it at 24 or 25, far above the 19 listed in the 3.0 MM). It's a reasonably powerful CR 23 now, and certainly much more powerful than either the pit fiend or the balor. 20th-level cleric spellcasting plus buff spell-like abilities plus regeneration, epic DR, high stats, fat natural armor, and spell resistance make for a tough customer.
#2, OTOH, is a bit more difficult for me to swallow. Most epic creatures have really, really high stats; the more to keep their skill checks and hit points up to par with challenges appropriate to epic PCs. I'd say the solar needs better stats across the board, and might propose +2 Dex, +4 Con, +2 Int, +4 Wis, and +4 Cha as reasonable modifiers to the base creature. (Personally, I also think the solar's weapons were downgraded a bit much; the old versions or something in between would be, IMHO, quite in line with the creature's power.)
