I didn't do mythics in legion but my guild did. The overall consensus is that +M15 in legion is pretty much +M10 in BFA, BFA is harder.
The higher the key is, the more team comp and spec come into play.
The number of total TIMED runs matters too, especially for certain dungeons.
I have a healer friend who is has a over 1k io score & 4/8 M, but he never did Siege before because he didn't have the attunement. Well we ran a 9 with him because he normally run 11s -14s, well we completely failed the key, he couldn't heal the last boss fight. We completely gave up after about 10 tries. He kept dying because he didn't know the mechanics and was OOM right away. It was more upsetting for him, then for us because we understood he never did it before.
Now this week is a healer check. The healer must try to keep the group at over 90% health when players are constantly taking damage due to sanguin. A healer who never ran a sanguin or grevious week will likely not be picked for +7 or higher keys.
Also, look up yourself on
https://raider.io