Well, explaining Interface-fails in short text is always a bit hard, but lets see.
The key F1 opens the help-widget, which is an area with text, located besides the editor. This area is just there, has now controls, no way to move it or close it. There is some text in it that explains this area can be closed with some shortcut, except it does not work. I don't even know whether it's a problem with my setup, because leo,has no way to check this. Just broken by design. Notable is that this hacked-in area seems to be used by several other functionalitys and plugins. All with the same fails.
Another strange quirk of leo is the minibuffer. It's something people knowing emacs will understand, for the rest, it,s just a strange line at the bottom of the window not explaing what it does, what purpose it has or how they can use it. But ok, that's how it is, just following some fade of unrelated culture. Really interessing is the usage. You enter some text, then hit tab to invoke autocompletion and surprise..in another part of the application, unconnected to the textwidget of the minibuffer switches a tab and some new widget appears presenting a list of possible candidates. Not that Qt hasen't a way to present autocomplete-lists, but thats just how it is. Quirky hackish, ugly. And of only working as ling as the tablist is not hidden.
Another funny part is the menu. Just look up what is weitten in them, what name, what order they have, and how this compares to the established names and positions in pretty much every other application.
Or let's take the configuration. Nodes with some directives in the outline. No actual documentation or discoverability in the app itself. No support to prevent failures. Not even instant-loading of changed settings. Restart the program, good enough 20 years ago, good enough today.
Or let's take the API: Single letter variable names. Objects with hundreds of unrelated methods and no documentation. Plugins which are just python-modules imported from the python-path, and not real plugins with actual structure and API.
Oh, and the worst, plugins which just don't work and functions which have no obvious result because someone forgot to refresh the outline. Though, this is probably just one of the endless number of obvious bugs one encounters at some point.
I'd be interested to gain some insight into why you find Leo's UX pure garbage and how it should have grown over the years.