Suppose I have a C program with multiple C threads calling Haskell functions, maybe the same Haskell function too, and the GHC RTS used is the unthreaded one. What will go wrong?

I ask because Cabal/cabal-install has recently added building of export-to-C shared libraries, i.e., the "foreign-library" section. It sees to all the technical details, very nice, I'm very happy about this addition, but it chooses the unthreaded GHC RTS.