Quantum-Classical Hybrid theories: How useful are they?

I have seen several attempts at building a consistent quantum-classical theory, i.e., a theory where a quantum (sub) system interacts with a classical one somehow.
Examples: (1) https://iopscience.iop.org/article/10.1088/1742-6596/442/1/012006.
(2) https://arxiv.org/abs/1802.04787

However, they tend to focus more on theoretical aspects and less on application to real systems. I guess, hybrid theories are supposed to give a good approximation in mesoscopic systems, where some things are big enough to be considered classical but not that big that all quantum effects can be ignored.

So, in what actual physical systems (not toy models) do quantum-classical theories give good results for practical calculations?