First of all, dragons are a flying troop. The game mechanics allow dragons to hit ground troops without being hit. That's why you bring troops such as wizards for support, to kill the dragon.

Dragons are not OP at all. They are rather slow and have less damage output compared to some other troops, such as wizards or balloons; these troops in turn have their own weaknesses (vulnerable to poison and really slow, respectively). Many clans choose dragons for their high hitpoints and ability to fly, two strengths that counter poison spells and ground-based armies.

Only clans with many TH8 and above players face the "poison problem": donating low health troops in the CC, such as wizards or just archers, will make bases vulnerable to attackers with the dark spell factory already built. Clans with TH7 and below players don't have to donate dragons to each other.