I have the BlueSC, and I find it frustrating for a couple of reasons. First of all, the rubber band that holds the magnet on the crank arm broke, and I lost the magnet. They sent a replacement - it broke too. I think it was just too tight of a fit.. I ended up getting a 3rd party magnet from eBay that I attached with a zip-tie.

The more serious issue is that it is finicky to keep adjusted. It used to be that I needed to tweak the position of the thing before every ride, and going over a bump would sometimes dislodge it and I would either need to stop and fix it, or just keep going. One suggestion that some have had is to attach the BlueSC with zip-ties to the bike frame and not use the rubber strap thing that doesn't hold things quite as securely.

Wahoo has two new sensors for speed and cadence that don't use magnets - I got those recently and my experience so far is that they are pretty foolproof. The speed sensor just wraps around the hub of one of the wheels, the cadence sensor is strapped to the inside of one of the crank arms.