I wonder why brl-cad never seems to get any attention in these sorts of threads. It's open source mature CAD that has seen some serious use over its lifetime.
brl-cad doesn't use the same paradigm as Fusion360, Inventor, Solidworks, etc. brl-cad uses Constructive Solid Geometry, similar to OpenSCAD, where you declare geometric shapes with certain dimensions to exist, and boolean them together into the final shape.
Most popular engineering CAD programs are parametric -- you lay out shapes in2D or 3D, and then specify their dimensions and relationships. Parametric modelling makes it much easier to create highly-interdependent geometry, and also to understand or explore the degrees of freedom your design does and does not have.
Also, modelling in brl-cad means typing into a command prompt [1], which makes it much more difficult to figure out what you can, or should, do with it.
