My three big problems with the retcon are as follows:

1. Time travel is for creating problems, not fixing them. I think one of the Back to the Future guys said that, or something. It's really hard to hold suspension of disbelief in a time travel story once you get to a point where time travel can be done at-will and history can be altered. Even if you show the problematic results of trying to do this, I'm almost never in a place where I can't just ask "why are you stopping? keep trying" when the hero inevitably gives up. When it DOES work, it feels cheap, because you basically could've just ignored the entire rest of the plot for this, in a lot of cases. Game Over un-happening makes the parts of it that DID happen feel like cheap shock value. In addition, we're now largely following a group of protagonists who experienced a totally-different three year journey from the ones we saw in game over, meaning our ability to relate to them and understand them as a collection of their experiences is massively hampered. "But ultimate selves" yeah okay man meet me when that actually matters.

2. Vriska Serket is for creating problems, not fixing them. This is just generally an issue I have with the writing post-Act 5 in general. Vriska is at her best when she fucking sucks. Even if she's trying to be good, even if not everything is her fault, the fact that she just keeps making everything worse is why she's compelling. There is nothing interesting about the ways she makes things better in act 6. She does not feel like someone who would draft a working plan for Collide that doesn't go awry, she doesn't feel like someone who would get everyone on the same page, and she super doesn't feel like someone who would...randomly force Rose to stop drinking booze. It's only here in Act 2 of Beyond Canon where I feel like any particular growth has happened where it makes sense for Vriska to be in-control and for that control to be well-utilized, and even that I can't be totally sure of.

3. That fuckass lilypad dialogue section went on for like a million hours