There is nothing really Mythic in Mythic 0, except that you need 20 more average item levels. Maybe in Mythic 3+ where the random affixes will fuck up 80% of the groups, things will be different.
Mythic dungeons will last one tier of raiding, then I suspect you'll suck with dungeon gear compared to raid gear, because set bonuses and trinkets are too important in modern WoW and they already said dungeons won't have any.
Broadening the discussion, I think WoD was fucked up for a ton of different reasons and Legion will be not different.
There is one in particular that is bothering me: I'm still wondering why I could get 650 ilvl stuff in Tanaan who could randomly proc as any item level up to 695 (stupid mechanic), but it was BoP instead of BoA, because only shipyard missions and bonus bags from groupfinder gave the BoA version, why everyone and their dogs could boost these to 695 then 705 with valor, when this completely invalidated HFC LFR loot, which was 685 useless pieces of shit loot, without anything that made it interesting, read set bonuses and trinkets. Please don't mention those special retard version of LFR-only set bonuses. It's like they thought "hey, let's make LFR raid loot useless before people even choose to enter, then let's cut off our dicks, two things that make perfect sense in our brain".
In Pandaria I could get 496 BoA tokens on the Timeless Isle which worked wonderfully for alts and I could get 528 gear in the SoO LFR raids (that's 32 fucking more item levels instead of 10 less item levels), which were slightly harder that HFC, so you couldn't truly sleep through them, rewarded LFR versions of the exact same shit that dropped in higher difficulty, including tier bonuses and trinkets. I actually had fun running a couple alts through SoO LFR, while doing HFC LFR is a gigantic waste of my time, unless I choose to endure farming legendary drops, which is soooooooo funny.
I won't mention Garrisons and Shipyards flaws again, but I'd have hoped that the new shit order hall wouldn't be mandatory, which won't be the case apparently, so we're again back at playing stupid facebook games in a MMO, or better a game that was a MMO some time ago and now it's not anymore. Given the premises, I'm not sure I'll last two whole months through Legion.