This is the Source.
These Pokemon are mainly banned because of very few energies to use devastating moves.
They can deal massive damage and have very high HP to what is actually needed.
For example, if you read this banned Slowking's moves, you will see that they can wipe the opponent out in almost any possible way.The attack is very weak, though it can confuse. But the Pokemon Power will basically just result in the failure of the opponents strategy.
I have played in a tournament before and I know how tough some (now banned) cards are.