Crystal is object-oriented, but isn't a pure object system. In a pure object system, literally everything is, or can be turned into an object. Smalltalk is the only other example of a language family that offers a pure object system.
With a pure object system everything is accessible by the system at runtime. It's ruby's biggest strength, but also the superpowers it grants aren't always used for good, which is why a lot of coders hate working in it. I only code for myself, my coding career is behind me, so I don't have to worry about other people's code.
Mruby's big downside is precisely that it's compiled, so the library base available is very small, and there's not much point in building on it, though a few projects have, such as DragonRuby. It also doesn't have a pure object system. Ruby has thousands of gems, mruby dozens, and you can see them all on this page: https://mruby.org/libraries/
With a pure object system everything is accessible by the system at runtime. It's ruby's biggest strength, but also the superpowers it grants aren't always used for good, which is why a lot of coders hate working in it. I only code for myself, my coding career is behind me, so I don't have to worry about other people's code.
Mruby's big downside is precisely that it's compiled, so the library base available is very small, and there's not much point in building on it, though a few projects have, such as DragonRuby. It also doesn't have a pure object system. Ruby has thousands of gems, mruby dozens, and you can see them all on this page: https://mruby.org/libraries/
https://dragonruby.org/
Static typing is also available in ruby but I understand it's clunky.