Arena is the only thing that matters when talking about balance.
It's not the same thing. Being able to play your character vs not being able to do anything no matter how good you are is a big difference. When me and Dreams play Mage/SP it never even matters who our opponents are; be it veterans or new players, it's all the same thing because of how insanely broken the comp is. Most teams don't even get to do anything because of all the CC.
When we played against Feral/Priest in WoD, arguably the most broken comp in that expansion, there were options, and your skill as a player still mattered because you weren't stuck in CC 24/7 against that comp. It was OP but it wasn't anywhere near this level of OP. Part of that was that you could change your gear to specifically deal with the comps you faced; as well as generally more abilities being available so you had more options there as well.
If we had all abilities from WoD, Mage/SP wouldn't be so bad. Not having half the buttons required to deal with comps like this makes it that much more broken.
I'm sure the battlegrounds are great fun if you're playing Mage.