I only did Shadowmoon Burning Grounds last night and took us over an hour. Sure we had one mate with ilevel 630 on his warlock which didn't help but this instance was not as easy. I had to go as heal (druid) and while before the patch i could almost fall asleep just hotting people up, I was struggling to keep people up with the damage (AoE) going out. Sure we were all having a laugh and if we would concentrate it would probably be easier but just before last boss, two adds cast "nova" kind of spell was so hard to heal thru. Each cast would do group wide damage of 150k+ .. considering my hots now heal for 6k/11k crit) that's a lot of hots required to top that off.
if we all would go on our main with 73x ilevel then it would be easier I am sure, but i still think Mythic dungeons THAT HAVE TON OF AOE are not as easy as some people say.
Mate of my had similar experience with his group (different guild with similar mythic progress to ours 5/13) and he said Everbloom was so easy but Shadowmoon they had to two heal as their main healer was well undergeared for it.
https://www.twitch.tv/golfik_/v/79113476 - mythic Shadowmoon Burning Grounds starts at around 1h 45m mark if you want to see (my druid is like 715 with first set piece). Tank is 715, warrior 733, first warlock 710 and one we are TRYING to boost is 640 or there abouts ....
I will go to other Mythic Dungeons today so will report later on the difficulty of them