Limitations

- No eval: eval, instance_eval, class_eval

- No metaprogramming: send, method_missing, define_method (dynamic)

- No threads: Thread, Mutex (Fiber is supported)

- No encoding: assumes UTF-8/ASCII

- No general lambda calculus: deeply nested -> x { } with [] calls

Assuming UTF-8/ASCII isn’t, IMO, a huge limitation, but some of the others likely are, for quite a few programs. Removing them also will require serious work, I think.