I'm not sure what you mean by "let you use mythics that were prior locked out of your build". Old mythics were nerfed pretty heavily and the few that are still good aren't really any better than they were before(I guess the ring can roll better now?). New unique mythic in theory are better than just uniques, but the odds of getting a good ones are atrociously bad.
Not only are mythics not that common to begin with, the pool is much larger now with no targetted farming(basically the pool is every single unique for your class+old mythics), and since they don't roll specific stats, you can get really shitty ones even if you luck out into exactly the few you can actually use for your build. Even when they roll perfect stats(or well perfect-1 since you can enchant one), they might still not be better than a well rolled GA unique because the unique affix buff isn't THAT much compared to GA on some mods. So effectively you need the right stats+some GAs for them to be always better than a non mythic, which is some quadratic layers of RNG shit fiesta. Meanwhile you can absolutely drop mythic uniques for your build that are just worse than a non ancestral unique or a GA legendary.
Also the crafting for them just sucks too, and you're capped to 1 crafted(which includes old mythics using the blacksmith/enchanter) and once you do get it with proper rolls, the crafting materials become entirely useless(granted with the amount of RNG you might not roll a perfect one in all season).
Overall very shitty system imo, they added chase items but forgot to make them worth the chase. Mythic uniques should have more guaranteed affixes(tbh they should have 4 perfect stat, and you can reroll one if your build needs something specific) and they should all be GA if they're gonna make it so they drop randomly. That or instead keep the current system but make them drop from their respective lair boss(that sucks for world pool uniques though but you can craft these I guess) so you can at least restrict the pool a lot more.
It doesn't really detract from the gearing system but it's entirely forgettable and it wouldn't be uncommon for someone to play the entire 2 months of the season and still not have good mythic uniques because they'd be a DPS loss over well rolled uniques instead.
As for the season implementation, it's pretty weak. It's fun in helltides and very good there, but in nightmare it's way too fucking slow and unrewarding and in pit it's even worse(although might be good for pushing because you can close rifts without having to kill the mobs).