Well, the UI is certainly a reason, but I don't think it's the only one. Most of the people who do complain about the game usually mention some or all of these points:
- bad interface
- bad game balance
- lack of character
- lack of difficulty
- technical issues
- boring lategame (trade routes + "nothing ever happens")
- things I may have forgotten
And I don't really see any reason to assume that these reasons not also apply to those who left the party silently.