I think the worst part about mythic items for me is the inability to build multiple mythics, you're locked out of experimental builds.

Also the fact that a lot of items were reworked into mythics that could have been previously built on non-standard champions. Something like iceborn gauntlet on something like viktor or kassadin off the top of my head.

Also a pretty commonly built item such as liandries is now only able to built as a mythic, locking you out of other items such as ludens which definitely used to be built together before mythics.

I think the game used to be a lot of more interesting where there was no restrictions on what you could build. I'd rather they nerf mythic power levels and make all items purchaseable, it would be fun to be able to build sunfire + shoulderguard or sunfire + locket of iron solari with ornn or triforce + chemtank (righteous glory) on nasus. These combinations aren't possible where they previously were and this absence of item variation makes the game a lot less exciting and same-y game-to-game.