After being able to play a couple of games, I wanted to write down my own thoughts, problems, etc I have, and just add another voice for Riot to hopefully hear. I've only managed to play through normal up to victory, i was only able to find one onslaught but we only got halfway through, so if somethings are different across the versions I don't know about it.
First off the fact that it goes to level 15 in normal and has a gold cap feels really limited, for Syndra especially since she'll only get half of her passive. Final builds are usually made up of 3-4 items, and then whatever component you could afford going into the final boss. I understand the finite gold idea, but it feels a bit limited in what you can get, especially since a few items can feel mandatory on certain characters.
Going back to Syndra, her ult is all about the big single target damage, but she can't use it on the boss, I believe Jinx's rocket is the same, passing the boss unless it hits an enemy ad nearby. Soraka's q also stood out to me, not being able to get the healing off the hit. I understand these are coded for champions and not monsters, but this is the Big Bad Final Boss, the only thing standing between the Guardians and Victory, it feels bad that a few of them are a little gimped.
Another issue i've found is the overlap of cc that the ads can chain. I've had games we're i'm suppressed one after another in a row or chain knocked up by reksai and cho before even hitting the ground, a little wiggle room would be nice, especially since the skarner ult chain can pull you far out of the safety zone or pull you and chill in a super velkoz laser. Speaking of Vel'koz he quickly turns the game into a bullet hell without any refinement, the split off projectile also seems to be nonexistent from what i've seen, it also seems the hit box can be a bit off, as i've been killed multiple times when i was not in the projectile path, I don't know if it was lag or just unlucky, but it stands out since it gets a little ridiculous. As far as the ads go, the only other thing i can think of is the om nom nom cho's, and how it feels bad to be eaten or watch someone get eaten, and know that the only thing you can do is whittle away at his health bar with no chance of freeing your comrade, only for them to be revived as soon as they died because everyone's standing on top of cho doing damage, maybe if the cho would spit whoever they ate out after dealing or taking a percentage of damage? This ones less of a complaint and more just looking at it like 'well that's just silly'
Lastly the boss, it's big, it's mean, it's for the most part juggling the ads and doing damage where you can. My only real issue is that the indicator for it's big dark sphere aoe attack doesn't give you much time to actually move, if it's in a corner far from any cover, your only hope is to use zhonya's if you bought it, or desperately try to flash past it. For the damage and one shot potential it has, it would be nice to have a little more time to react before getting obliterated.
In closing, I listed a bunch of complaints, but overall I genuinely really enjoy this game mode. The last few new modes haven't really caught my attention, but I love PvE and the Star Guardian skin line in general. These are just a few things that I noticed in the 20-ish games i've played.