Sigils are a moronic concept, as for their balance in the game... they don't help one character that much more than another. Sigils do, however, highlight one of the key issues in the game: the Demigods with the highest health are generally the best. The flag *exploit* is not realy an exploit, it's more of a tactic. It's such a minor thing too. You can only push the gold flag with towers up if your opponents have screwed up.
Mines are absolutely not overpowered. The only thing regulus has going for him is that he's strong in group fights because of mines. They can be annoying, but they can be avoided and destroyed by minions.