Don't really get all this hate about the new engine. It was indeed not the best decision to change it, I agree, but it just happens to be on the initial releases of a game, that it is buggy.
Most hate
is due to the change, that you acknowledged as "not the best decision".
Most people don't even know what's an "engine", let alone discuss technical [de]merits of Godot or Ren'Py; however since
this is not the initial release of the game, they can compare how it was in 0.8.1 versus 0.9 and say that the game is worse: older saves don't work, basic functionality (as text skipping) is bugged, the game performs considerably worse in comparison with earlier versions and games with a similar functionality.
And some already caught up that unnecessary mid-project changes are often a symptom of delaying development on purpose, to milk the source of revenue (in this case, Patreon). I am not sure if that is the case here, but the odds of that happening are not zero.
To add injury, as people correctly highlighted that Noriko's "femdom route" is not femdom at all, the dev's answer was effectively calling people stupid by proxy with an "ackshyually my definishun of famdam".
Give this a read for further info - that poster explained it better than I could.
It's also relevant to note that the dev's "dose are pirares anyway than they're critizism is invalid" pissed off a lot of people. Some because the dev is right (paying customers should be prioritised over pirates, even if the later aren't simply "leeches" as some believe); some because the dev is wrong (brushing off criticism based on who voices it is a fallacy/stupidity called ad hominem).