It doesn't have to be a mess, especially when using Unity was already overkill in the first place. It's a simple VN that is much better suited for Ren'py than Unity. However it feels like the code was thrown into a script converter and just converted to Python instead of being tailored to Ren'py itself. This caused makes it so the game uses it's old custom Unity functions instead of Ren'py's built in ones.yes, changing the game engine mid dev is a bad idea.
I hope other developers will learn from the mistake.
So basically, the game is better suited for Ren'py but has been poorly ported. From what I've seen of the code at least.