My main issue with the mode thus far is that it throws too many things at you at once, so fights quickly turn into absolute messes whose outcome feels out of your control. I'm fine with getting punished for not dodging a Vel'Koz projectile, not paying attention to the Rift Herald's bullet hell, getting too close to Skarner, Cho or Kha'Zix, but when there are literally over a dozen other monsters who all command your attention, and punish you heavily for not doing so, the end result just feels like you're getting punished all the time for no good reason at all. I think part of the issue here is that enemies feel like they're being randomly distributed throughout missions, so whichever challenge was crafted around them individually collapses when combined in unexpected ways, and the end result is that some missions just throw unwinnable situations at you and end your run, without the players having any real say in the matter.
From what I've seen, another big issue here is that mages fall off very quickly in this mode. Monsters end up having too much health to be killed in a single spell rotation, and mages turn into sitting ducks when their abilities are on cooldown, which leaves marksmen as the only champions able to consistently clear levels. Lux, Ahri and Syndra all have crowd control that isn't particularly well-equipped to deal with multiple enemies coming from all sides, and even Poppy, Lulu, Soraka and Janna tend to only be able to disable a couple of monsters at once, which makes it hard to deal with stuff like Kog'Maw packs or droves of Khas or Rek'Sais, whose counterplay is meant to involve stopping them before they can deal major damage.
I think the game mode could've benefited a lot more from more hand-crafted enemy setups and challenges: at the very least, I feel right now the game can throw situations at the player that simply cannot be adequately dealt with, and it would help to add some controls to enemy distributions that would avoid certain unpleasant combinations. Right now, much of the difficulty feels like it was a good idea on paper, or in isolation, but frustrating in practice.