I'm not sure, I think Solvespace is history based, but I don't know enough about CAD geometry engines to give an in-depth answer. Most of my experience with CG comes from the simulation modeling aspect (think more GEANT instead of CAD).

I'm pretty sure Solvespace is using their own geometry engine implementation and not something like OpenCASCADE or CGAL (as in OpenSCAD). As you note, it really struggles with curved surfaces. Just drawing a cylinder and doing some booleans on it will cause problems. And it can have a fit with constraints too, e.g. the symmetry constraint almost always leads to an unsolvable sketch for me. Onshape and Solidworks (which I dislike) perform noticeably better.

But, Solvespace really in most cases gets the UI and workflow right. It's highly intuitive except for a few things (as I said, a couple things like changing colors for a solid are really irritating). I think it's in most cases a better choice to learn for hobbyists than something like OpenSCAD, which is the maker darling.

OnShape to me feels like what I want Solvespace to grow into. The workflow is actually very similar, but as you say the geometry engine is vastly superior. And it's obviously the money thing. CAD is very much a "long tail" process, where it's not hard to make 90% of the features that are needed, but where everybody needs lots of stuff from that last 10%. This has always been an area where open source has had trouble.