I suspect it's the ways in which they are a bit crap. All software is a bit crap one way or another.
FreeCAD's UI is "a bit crap". The "workbench" metaphor is fine as a metaphor, but the specific way the workbenches are put together in FreeCAD is oriented more towards the way the functionality is implemented than what the user wants to do with it. You have no choice but to understand technical implementation details.
That's just one example, but that attitude to UI in FreeCAD is absolutely pervasive.
I think that's a general problem in the space. The user interfaces in general aren't designed, they're an outgrowth of the direct implementation of the underlying functionality.
FreeCAD's UI is "a bit crap". The "workbench" metaphor is fine as a metaphor, but the specific way the workbenches are put together in FreeCAD is oriented more towards the way the functionality is implemented than what the user wants to do with it. You have no choice but to understand technical implementation details.
That's just one example, but that attitude to UI in FreeCAD is absolutely pervasive.
I think that's a general problem in the space. The user interfaces in general aren't designed, they're an outgrowth of the direct implementation of the underlying functionality.