(Disclaimer: physics postdoc working in an unrelated area of research, not a domain expert.) I'm assuming you mean GR and the Standard Model (which is a very l

(Disclaimer: physics postdoc working in an unrelated area of research, not a domain expert.) I'm assuming you mean GR and the Standard Model (which is a very long way away from QM). If so, there's a number of known issues with the SM, but that's not really what unification is about. Unification is about the simple fact that given the four known fundamental interactions we have a theory that (largely) accounts for three of them and a theory that accounts for one, but the two theories are mutually incompatible. That's an issue because whenever we encounter phenomena where gravity and at least one of the other fundamental interactions are simultaneously relevant, we have no established theory to describe them. Such phenomena, while comparatively rare, exist and are quite important to our understanding of the universe. Hence the long standing desire to produce a unified theory.