V-brakes and cantilever brakes pull a different amount of cable- if you're using the original canti-brake levers with v-brakes, then the lever pulls only about half the cable it needs to to operate the brakes properly. The brakes will be very hard to adjust for pad clearance from the rim, and will feel really spongy. You'll need a pair of "travel agents" to convert the cable pull ratio, or you'll need to change to v-brake type levers.

But, properly set up canti brakes are exactly as powerful and reliable as v-brakes. After all, they're mounted on the same mounts, and their pads are the same size. If you want more leverage, then shorten the straddle cable, but it comes with the cost of less adjustment room and clearance.