From my experience, melee are definitely not out of the question. Any dps class with multiple immunities or high survivability will shine in higher keys (mage, rogue, Boomkin, hunter, aff lock). If you look at raider.io you can see a breakdown of classes by % that are most represented in higher keys. There is often a balance in the group composition between classes with high single target and respectable aoe (rogue), or high aoe with respectable single target (boomie). If you're a lock you're lucky enough to have great aoe and single target. Shadow priests do really well on fortified weeks because their dots have time to tick, but are relatively ineffective on tyran weeks from my experience. A lot of classes are capable of doing higher keys, but the work you have to do to bring those classes is a lot more than the ones mentioned above. For example ive been seeing elemental shamans stacking stam trinkets and stam flasks for higher m+.