I never understood the "git cli sucks" thing until I used jj. The thing is, git's great, but it was also grown, over time, and that means that there's some amount of incoherence.
Furthermore, it's a leaky abstraction, that is, some commands only make sense if you grok the underlying model. See the perennial complaints about how 'git checkout' does more than one thing. It doesn't. But only if you understand the underlying model. If you think about it from a workflow perspective, it feels inconsistent. Hence why newer commands (like git switch) speak to the workflow, not to the model.
Furthermore, some features just feel tacked on. Take stashing, for example. These are pseudo-commits, that exist outside of your real commit graph. As a feature, it doesn't feel well integrated into the rest of git.
Rebasing is continually re-applying `git am`. This is elegant in a UNIXy way, but is annoying in a usability way. It's slow, because it goes through the filesystem to do its job. It forces you to deal with conflicts right away, because git has no way of modelling conflicts in its data model.
Basically, git's underlying model is great, but not perfect, and its CLI was grown, not designed. As such, it has weird rough edges. But that doesn't mean it's a bad tool. It's a pretty darn good one. But you can say that it is while acknowledging that it does have shortcomings.
It's more than that; it's also git's incredibly unfriendly way of naming things.
Take for example the "index" which is actually a useful thing with a bad name. Most tutorials start by explaining that the index is a staging area on which you craft your commit. Then why is it called index and not staging area? Incredibly bad name right there from the get go. If you ask what the word "index" means in computer science, people usually think of indices into an array, or something like a search index that enables faster searching. Git's index doesn't do any of that.
And git's model leaks so much implementation detail that many people mistake these for essential concepts; there are people who would tell you any version control system that doesn't have the "index" is not worth using because they don't allow one to craft beautiful commits. That's patently false as shown by jj and hg. This useful concept with a bad name becomes one amorphous thing that people cannot see past.
