A huge bummer about Guile (and that is frankly shocking for a language designed to be embeddable) is that it relies on a bunch of global state, which means that you can't embed multiple interpreters a single process. This totally kills it as a embedded scripting language for many applications, and it's also just bad practice in general. Even Python, far less easily embeddable, supports multiple interpreters, though before Python 3.13 (I think) they all shared the GIL.

Aside from this issue, the Guile API is pretty good, but the best one by far is Lua's C API, which is just a thing of beauty. I'm curious to try it with Tcl.