Git - GUI vs CLI - The Devil's Advocate

I quite often hear experienced Git users say that you should always use the command line for Git, and you should try to avoid using Git GUIs. Whilst I understand the reasons given, I also think it's a huge generalization that unfairly sees Git GUI users as unlikely to really understand Git properly. I certainly know Git very well, and honestly think I'm far faster and more efficient in a Git GUI than I could possibly be on the command line - and I'm certainly not slow in the CLI.