Current implementation is filled with bugs, which is a real shame because in many ways it is better than Lua: the language is better (for me), and the C API is much simpler.
I've not run into too many, you just have to be careful around re-entrancy from Wren->C->Wren, and slot allocation in foreign constructors.
There's some big stuff in the pipeline, so I'm sure it'll be enhanced soon enough.