> Guilds would have a major performance problem: can't allocate objects without GIL.

Why not? It'd be possible to have TLABs, isn't it? But yes, GC would still be for all Guilds at once.

Racket places don't allow to share objects, only arrays of primitive types, which seems very restrictive. And Racket futures are even more restricted.

I'm not sure about Guilds since it's not there yet, but it already sounds closer to Ruby multithreading and more flexible to me from an usage point of view.