It is banned in schools. As I said in another comment, that outlaws it for the vast majority of players at the place they used to play it.

Believe it or not adults playing conkers or people playing conkers outside of schools isn't a common pass time.

It is pretty much RNG, though you can massively nerf a conkers structural integrity by making the hole through the middle poorly, so there are some techniques. People also used to use thicker shoelaces like in vans, which I think made the centre more solid. I've never run an experiment to verify the difference that might make.