All things being equal, a flat bar lever should be no different from a drop bar lever. Any difference in leverage will mostly be in the ergonomics of the hand position.

If pulling drop bar levers from the hoods, you don't have that much leverage because you're moving the lever from near its pivot point. It's like using a long wrench but grabbing it near the jaws. Pulling the levers from the drops, on the other hand, gives you tons of leverage. On a flat bar, you are grabbing the whole length of the lever so you have good leverage.

Cantis can be very powerful if set up correctly. Some are easier to set up than others. There are more variables to adjust with a canti because you have the straddle cable position to consider. V brakes and sidepulls are usually easier to set up.