Cleared phase 1, 2, and tested phase 3 for a good while. Was a full group on voice with some people with fully perfect optimized gear rolls(not sure exactly how many).
Generally everything had ALOT of hp and it feels a tiny bit overtuned in that department.
P1 felt doable if everyone played perfectly with minimal deaths and even then it came down to kitting the final dragon while it was berserk.
P2: oh boy vergos. Hes got over 120? Billion HP so this takes way longer which is fine. People died alot mostly to some people being new to harrowhold. Our P2 took 7:23 to push to 70%.
P2 Ballista phase: I'm not sure if its undertuned on purpose or bugged for test server as it took 23 seconds to die. 3 people properly doing ballistas did 75 million damage and then it ended. Fine either way since getting to this part takes WAY longer than the normal harrowhold.
P3: This is where the massive hp becomes a bit of a problem. The 10 minute timer starts at 90% so you need to deal 96 billion damage in 10 minutes, 9.6 bil per minute, average 13 mil dps per dps. You would need ZERO deaths WITH perfectly rerolled gear on every single player to beat this. Maybe its possible but i highly doubt it.
P4 would run into a similar issue with the 10 minute timer.
Here is the vod for the run starting mostly at p2: