That's not saying much. So many other design decisions* in 5e itself make the addition of magic items like adding steroids to flag football players.Yes, I think magic items throw off the monster challenge math more than any other factor. I believe the game is balanced on the assumption that the PCs have no magic items, so as they accumulate more, the published encounter guidelines get less useful.
* Take your pick... turbo charged rests, removal of negative hitpoints in favor of an set of absorb shields that only takes 1 point of healing to fully recharge to soak the next hit up to max-1hp, ranged 1st level heals, etc.