It's fine. A garmin speed-cadence sensor is 100% safe and works extremely well when mounted to rear wheel. The magnet won't fly off and even if it did, wont' break anything.

The speed/distance on your trainer won't match real world numbers. It might be close with a good trainer, but it might not be. Just imagine - if the trainer resistance is way too high compared to a real world, you'll have way low distance numbers. And vice versa if it has zero resistance- you'll go 30mph barely breaking a sweat.

That said, I definitely disagree with all those above who say it's useless. Having the distance on your workout can be extremely useful. Total distance covered on steady state efforts gives a pretty decent estimate of total work you've put in, so as long as trainer settings are constant, you're clearly improving if you can go 60,90, 120, etc minutes as you get stronger. As well, trainer speed is a very useful surrogate of power on most trainers. Meaning that you can use your trainer speed as a gauge of how hard you're working at any moment - very useful for things like hard intervals or even long steady state efforts. Again, all the data is relative to prior efforts - if your speed on the interval is getting better over time, you're getting stronger. But you wont' be able to compare to others.

I spent a year and a half without power and using only a speed/cadence sensor for serious indoor training, and it was undoubtedly a key addition to the workouts, being able to track speed on my intervals (worked even better in conjunction with HR). You'd be surprised how with just riding with perceived effort, your power can drop off especially on long rides - with the speed sensor you'll see that dropoff immediately and can even find the point where you have aerobic decoupling (power drops off a lot for stable HR - Friel makes a big deal of this point).

(I'll also anticipate now that some powermeter fan will reply to this post pointing out how useless these metrics are compared to a powermeter. Yes, that's true - I now train with a powermeter, and it does all of the above, better and reproducibly across different users. Still doesn't mean a speed sensor is useless; if i didn't have the means to own a powermeter, the first thing I would get as a serious indoor trainer cyclist would be a speed cadence sensor to get some objective numbers on my training.)