I haven't found anyone who is what I would call weak. Some Demigods are weaker against specific others, but overall they all have the capability to kill, destroy, and dominate pretty equally.
Anyone who thinks that Unclean is weak needs to learn to play him. His poison is hard to remedy, most characters can't counter it. Early on it will kill you even when you run away.
Rook needs time to get going. His towers of light are the main key to using him effectively.
Oak is godlike everytime I use him. Most people assume that as a general he needs a ton of minions, not really true. Spirits make him deal more damage, his ability to heal from kills shines with his area effect attack. Just get him some mana regen and he can live forever if you aren't foolish with him.
Regulus is a demigod killer, plain and simple. But you can kill towers solo if you use his range. He can effectively kill them and take no damage from them.
Sedna is my favorite, and I have every achievement with her. Yeti's are her strongest power. Nothing screens and most demigod's ignore pets, so they deal some great damage to the moron who ignores them. Her healing and regen are so high that she can take serious damage long enough for her yeti to beat the opponent down. If you aren't playing with yeti's you shouldn't be playing Sedna.
Anyway, there are plenty of gear choices to balance the weaknesses of the demigods, making them able to fight anyone overall. There will always be some imbalance in certain cases, but I haven't seen a player that I couldn't touch with any character. You just have to remember to play to yoru strengths and exploit the enemy weaknesses...which means you need to play each demigod a little to understand them. Some have an easier time in some roles, but they can really do it all given the right strategy.
