The UI seemed to have a lot of modes. I mean quite literally they seem hard to learn.
Come to think of it, Blender has a very nonintuitive UI as well. Holy moly, yikes.
I like to look at Solidworks as an example of how to make a 3D user interface rather approachable. It has simple objects organized cleanly.
Granted Solidworks is mechanical and ZBrush is organic, but I can't fathom that organic modeling would require a more complicated UI than mechanical design...
A command-line program is not typically intuitive to the average person, it's why we have GUIs, but given enough knowledge of how it works it's very efficient and powerful.
But just because it runs in a terminal doesn't mean it's scriptable easily.
I kind of think that once you've used ncurses in a terminal to make something interactive, you've just made a bad GUI. Slow, often bad scroll behavior, and very prone to the terminal state getting corrupted. (Think of what random binary data will do to your terminal...)
But that's just my opinion
They've redone it in the latest version (2.8) and fixed some of the oddities. Probably the most obvious example being left click selects instead of placing the 3d cursor, so it now behaves like every other program.