I said it. When I was playing WoW I first experienced M+ when I joined a M0. Didn't think it was that bad but then once I got up around 8-10 I noticed finding groups became painful. Bare in mind I was a frost mage and at the start of 9.0 after pre-launch nerfs Frost Mages were dookie. Fire wasn't but man I really really hate twitchy gameplay.
Being honest here--there's no good logical reason from my perspective as to why this doesn't happen. I know people will all say comp comp comp--my response is that the burden of responsibility for balancing the game so that all specs, and all classes are viable with one another is a dev issue, and any deviation from that statement is you as a paying customer being okay with a subpar product.
Hire some actuaries to balance your numbers and let players connect with one another automatically. The insistence on Premade grouping is so dated and not helpful to anyone.