>> SolveSpace's kernel seems rock solid to me. It just doesn't support fillets or bevels.
Sorry, I work on Solvespace and can confirm it has some serious NURBS bugs. There are several open issues on github relating to NURBS boolean failures. The problems seem to fall into 4 or 5 categories, and we can probably fix some of the common ones. It's going to take time and some deep dives into the code though.
I guess fillets are probably one of the most complex things for a CAD tool to support.