I've been using Signals for about 2 years now and absolutely love it. Yes, it does indeed have magnets on the back (strong ones!) so you can mount it to the front of your pellet hopper or under the shelf, and as it is angled slightly up, it's actually easier to read than the stock Traeger LCD display (I've got the Pro 780). As for water resistance, it is rated at IP66. I wouldn't leave it out in a constant downpour, but it will be fine in a light rain or a splash. Basically, if I wouldn't leave my Traeger plugged in and running in it, I wouldn't leave my Signals out in it either.
As for pros, range is excellent (I highly recommend just using it on your WiFi). You have to connect via Bluetooth initially, and you have to be connected to Bluetooth to do firmware updates. Speaking of which, it is updated frequently - Thermoworks support for Signals (and their mobile apps) is excellent. Stock probes are commercial grade and good quality, but they aren't water proof (meaning, don't submerge them). I always keep an extra set laying around. All in all, I highly recommend Signals. The only other one I'd consider would be something from FireBoard.
About Meater - they're nice in theory; I've owned a set (second gen). The batteries cannot be replaced, so once they go out - they're trash. App is kinda' slick, but the range of the actual Meater probes (which require a Bluetooth base station) are pretty crappy IMHO. I also worry about their temp range - it's easy to destroy a Meater probe if they get too hot (though that's unlikely in most Traeger). I'd rate Meater at the bottom of the three choices being discussed.