As a shorthand "quantum effects" are often hand-waved as crazy powerful "weird" things, but we can certainly model them and they really aren't as "magic" as a lot of popular science would have one believe (as much as even some of us otherwise rational people so strongly wish to believe in quantum magic).
Our ability to model quantum systems is feeble indeed. Anything more than about 40 qubits and it starts costing millions of dollars. 50 qubits just forget about it.
There's plenty of quantum condensed matter physics that is just completely impossible to model.
Even in chemistry, there are huge swathes of molecules that we fail to be able to model. This is hopefully going to be the "killer app" of quantum computers, btw.
As a shorthand "quantum effects" are often hand-waved as crazy powerful "weird" things, but we can certainly model them and they really aren't as "magic" as a lot of popular science would have one believe (as much as even some of us otherwise rational people so strongly wish to believe in quantum magic).