Just to add to this, fillets may seem trivial but they are a good example of the difficulties in parametric cad.

Fillets are added to edges, edges can be formed both by a single feature (say an extruded 2d shape) or through the interaction of two or more other features (think of two extrusions 90deg to each other interacting and forming an edge). They can also link together or become unlinked via changes.

Referencing these edges, and keeping track of that reference through changes to the parameters is fundamental to parametric cad. It's one of the areas that OpenCascade struggles with.