proving grounds owns and will quickly point out your weaknesses to you if you weren't already aware of them. i had a lot of fun doing endless 30 for each role. my biggest complaint is that a mistake or lag spike can undo many minutes of effort, for instance i wiped 4-5 times on wave 26 healer while ironing out my CD rotation and talent choices, and 1 of those was a lag spike which was extremely frustrating.
e: also want to note that healer rng seems especially punishing, since bad bleeds or debuffs on the wrong targets can mess up your CD management. also the npcs are sometimes total shit at interrupts, thankfully i did it as a resto shaman so i had grounding/wind shear/cap totem to control the casters but i pity healing classes with less caster control (priests' lack of true interrupt seems pretty threatening)