I'm guessing that you want this for a winch for climbing. We had a purely climbing robot in 2013. Video
here, but skip to the middle of the match to see the actual climbing. We used the WormBox for our first competition. Our robot weighed about 130 lb with bumpers and battery. We had no issues with backdrive at all.
We did have issues with the plastic gears in the WormBox. It doesn't take much at all to break those gears. If we accidentally bottomed out the climbing mechanism, the gears would strip instantly. We went through three sets during our first competition. We switched to metal gears after the first competition, and we had no other issues in the next two competitions.
I would recommend going metal if you want to use a worm gearbox for climbing.