just what i've said in the post. deck syncing is error prone, card creation can be tedious and unintuitive for most, and general ui is dated among other issues.
surprised to hear you think the GUI is fine. I've been using it for a year, and I know my way around the app & menus by now, but I still struggle on occasion, as things are just not intuitively laid out and/or not well-presented.
Oh, well everyday usage is totally fine (reviewing cards, adding cards), but some things are still a little weird, card types vs note types for example, but that comes up rarely enough that it doesn't bother me.
Maybe I'm just tired of apps that get more and more bloated and slower year after year, while the ui gets changed to "simplify" things and become less usable as a result.
But anki has mostly stayed the same, it's fast, responsive, does what I need it to do, almost never crashes, etc. It's also been a daily routine to use for literally years, so it's sort of soothing. Yeah it has some rough edges, but they don't come up very often.