I am playing the game on PS5, on difficulty POTD all upscale. I am about to finish the game with all DLCs, by having 150+ hours in the game. So far I did all the fights, every single optional ones, including all the encounters of the SSS DLC. I have not faced any major game breaking bugs and on PS5 loading times are also totally fine. Only the The Lagufaeth Brood fight in SSS crashed time to time again, but even that I could overcome by keep trying for 3-4 times and it was done.
So I started the mega bosses, Sigilmaster Auranic first and encountered the craziest bug. Randomly (but somehow linked to the time when Auranic activates a totem), two of my party members became unconscious (health 0, nothing in Action Log about this). It is always the same two members, it my party it is my Seer and Celebrant. First I thought it might happen only once and gave Ring of Reset to my Celebrant and once he was back, he could revive my Seer. Hell no, this does not happen only once...now I managed to beat it and I can tell these two members would die 15+ times in this fight, without having any chance to avoid it.
But I did it ! Nothing too crazy unknown tactic, but still you need a lot micro management. It is the well known Barring Death's Door + Salvation of Time + Brilliant, this the only way to survive. Barring Death's Door on the Seer and Celebrant, while giving Brilliant inspiration also to the Celebrant, by using Seer's Ancestor's memory, afterwords SoT. Still you need to focus to get Obelisk of Cleansing down as fast as possible (Minoletta's Missile Salvo I recommend, by also giving Ancestor's memory to your wizard to refill). On the top you could be hit by Arcane Dampener too even from Auranic's summons. Also try to be ready of the lot of Stun and Paralyzed effect, when you cannot cast Salvation of Time.
The last update on Robes of the Weyc could also help, but that is bugged on console too...: https://forums.obsidian.net/topic/119570-second-update-in-robes-of-the-weyc-problem/
So all in all, it is possible, but well, not easy