Just had a look at ROAR rule book and sensorless is legal for all electric classes. They did change the sensored only rule for 1/8 a few years back but still required sensored for 1/10 last I checked. Obviously no longer the case making the Traxxas motor legal. Sensored is smoother in the application of both throttle and brake not just down low but all over. It is an advantage if your throttle finger can use it, if you have an itchy trigger finger it's not gonna help you.
There are still a myriad of rules regarding motors in short course truck classes that MAY vary track by track. Some may only allow 540 size motors, some may allow 550 or other size motors.
There's more to choose from in 4x4 act these days but plenty do actually race Slash 4x4's. They just aren't very stock when the owner is serious about placing up at the top. Aftermarket motor/esc, shocks, driveshafts, chassis, etc. By the time you add it all up the "race" truck competition doesn't look as expensive, and as a benefit you can more easily imitate another person's setup. Got a Durango or AE? Go to their websites and lookup setup sheets or just ask for tips at the track from someone with the same truck. With a Slash 4x4 if you don't have the same aftermarket shocks and other parts the setup from another racer is useless to you leaving you on your own for a starting point. What parts the track or local shop has in stock should be considered as well.