The engine itself is mature and capable. In the end all of the implementations in it will most likely be rewritten by yourself because that’s life, not a fault of an engine.
Unity has an asset store, but I’ve never come across a problem because I write my own stuff - that’s just how I roll. Most will anyway in their own game.
In the end, use whatever is the most fun for you. That will be huuugely more important in the years to come.
In any engine, glsl will be a steady diet, again - all engines are running the same instructions - so a non-comparison.
Moral of the story: enjoy yourself