I'm a pretty cynical guy but in this case I disagree, the vast majority of UIs are simple enough that you just don't notice them working. Having tried occasionally to bake a UI myself, We take good UI for granted.
Yeah there's a learning curve but by and large even across operating system there are consistencies and even technically illiterate users can get up to speed quickly.
Do you remember the first time you discovered you could swipe in from the right in windows, or down from the top in android to access hidden functionality? Or the magic swipe to go back in the browser? Have you tried accessing the internet from small rural town USA? If you leave the coasts there are places you can't get cell service, much less blazing fast cell based internet from a rural provider. Those coverage maps the providers show in their advertisements are huge lies. Modern web UI's don't tend to run great on HugesNet, or single bar 3G.
That is just the tip, UI's have majorly regressed from the time period of the original iphone. Go to the local senior citizens home and give a bunch of them modern android/iphones and see how many can figure out how to add a contact, or send an email. Worse companies like consumer cellular have a section for flip phones. Not that it is really bad. My mother has one and likes it more than any smart phone, but its limits her.
Yeah there's a learning curve but by and large even across operating system there are consistencies and even technically illiterate users can get up to speed quickly.