> eval, send, method_missing, define_method , as a non-rubyist how common are these in real-world code?

Quite a lot, that's what allows you to build something like Rails with magic sprinkled all around. I'm not 100% sure, but probably the untyped JSON ingestion example uses those.

Remove that, and you have a very compact and readable language that is less strongly typed than Crystal but less metaprogrammable than official Ruby. So I think it has quite a lot of potential but time will tell.