Canada

I know that nuclear and hydro can constantly cover it, the point is that when it's very sunny out countries with good solar adoption will already 100% cover it (if not more). The nuclear power at those times has to compete with cheaper solar power, which it loses on price. And because the grid can't handle more supply than demand, it requires shutting something off. The cheapest power is solar so you'd prefer to keep that on for economic reasons, but since nuclear is bad at scaling up and down you have to pick the more expensive option. This increases energy prices beyond what is really necessary.

This also becomes even less tenable as battery adoption increases.