Vimscript sucks, using Lua will make it easier for people to contribute I think.
It's not a super-popular language and makes some things harder than necessary. Base 1 arrays means you are almost certain to bake in bugs accidentally. Meta tables are even worse though. They result in many subtly different object systems. Maybe that's not as bad in a unified project, but it's the path to a fractured ecosystem. The builtin string library is very barebones which isn't a great place to be in a text editor. Finally, the "regex" match function is hyper limited.
I'd argue that either Python or JavaScript would be better.
The OOP through meta tables is bit more interesting discussion. Meta tables also enable other dispatching mechanisms, if you want to get fancy. In my case I find it makes me reach for OOP less often, which in turn reduces code complexity. I agree that it's frustrating when different libs are each using own OOP implementation. It doesn't matter too much because all those implementations are light and don't pollute the code base. Fractured ecosystem is both good and bad thing.
The string handling is kind of a pain point, especially if you are spoiled by Python. I have no idea how Neovim + Lua handle Unicode text?
Python is huge & complex language, as well as 10x slower than Lua. JS has more rough edges due to historical baggage, also a fast VM would be much much harder to integrate than Lua. I'd say they would both be worse choices for maintainability and Neovim usability.
JS has better string builtins than Python IMO.
QuickJS is made for embedding and has similar performance to Lua (slightly slower). I don't think you'd want to embed a large VM like v8 anyway.