As someone who has only been successful Mythic raiding in Legion (not for lack of trying), I will say in my experience, that our BIGGEST problem that held us back from completing a tier on Mythic on time before now was the static 20-man raid size. Obtaining 20 Mythic level raiders that are:
1. Skilled enough
2. Available enough
3. Consistent enough
3. Mature enough
4. Able to play with others
...IS HARD. VERY, VERY HARD. Harder than the bosses themselves.
Granted, we're not world or server-first level players. We don't want to be. But finding people that don't jump ship or burn out was single-handedly the most difficult obstacle I achieved in WoW. I'm not sure if that's by design, but having 20 doesn't even work. What if someone gets sick? What if an encounter needs a different class to help smooth it out? You gotta realistically have 23-25 Mythic raiders, 5 of whom who are willing to sit during progression.
I just don't like this system. It excludes people from content who would otherwise LOVE to do it. I just can't buy that it can't be difficult AND flexible. Like, take Heroic difficulty, make it 500% harder, and call it Mythic. Voila. Lol, I kid, I kid.
Anyway, rant over...