Caelum has strengths but to say that they are too strong is just panic. Most nations have some strengths. It's how you play the nation that makes them strong. I have seen games where Caelum is destroyed in the early game, games where Caelum is destroyed in the mid-game, and hopefully soon a game where Caelum is about to be destroyed in the late-game (by me - LOL).
Anyway, Abysia has great strengths as well. Some people think their blood magic is almost unstoppable if they survive to middle game. I know that's not true.
Some people think Ermor is unstoppable all the time. I know that's not true.
Some people think Arco is unstoppable in mid-late game. I disagree.
Some people think Vanheim is overpowered throughout the game. I disagree.
Some people think Jotunheim is too powerful early and mid-game in particular. I disagree.
Ulm is very very strong early in the game.
I could go on and on, but I do not believe Caelum is overpowered...it depends on how good the player is and whether the player plays to the nation's strengths...any nation can be powerful if played correctly.
Regds,
John