BRL-CAD is one of the few "full-fledged" CAD systems that I know of which is explicitly CSG, while most (all?) commercial CAD programs are B-REP in nature.
CSG and B-REP are two worlds apart. I'm not sure why in the comments we're suggesting alternatives in the two categories interchangeably. They're not [*].
An alternative to BRL-CAD would be OpenSCAD or libfive studio + other tools (since BRL-CAD does a ton more) and more recently CadQuery. Maaaaybe Tinkercad could be classified as CSG?
CSG makes some operations easy at the expense of other things that become incredibly hard. The same is true for B-REP. Doing gears, highly procedural or FEM-directed models via CSG? Easy-peasy. But try to add a filled between two edges. Step it up 50 notches to fillet one edge and do the equivalent of "tangent propagation". Now the same, but also try projecting a structure on that round face.
CSG's speed is also wildly overblown. While B-REP is not exactly lightweight, computation complexity scales with the complexity of the model but can be sped up by caching intermediate representations.
In CSG this is not always true, since caching the intermediate representation can destroy the accuracy of the final mesh in ways which are very difficult to compute. So even for apparently easy models, times spend walking the space to remesh your model can grow _very_ quickly, hampering the ability to iterate quickly and experiment.
Not to mention, "measuring things" in CSG is hard. This is never mentioned, but none of the tools I've worked with so far offers convenient ways to examine and measure the objects you're working on. For a parametric model this is vital!
[*] Not saying there's no overlap. I'm using both systems. B-REP generally wins on a model which is feature-directed, which is why it's more popular. But I can relate to people that think that BRL-CAD is "hard".