Youtube for Instance has changed it's layout to try and reserve horizontal space so it's "compatible with mobiles". But the thing is, there's a Youtube app for phones and has nothing to do with the PC site ! And when you open Youtube with a browser app, you still get a separate mobile interface. What were they thinking? It's like the design team had no idea about the mobile app.
There's a saying,
"if it ain't broken, don't fix it",a thing that many companies don't understand and often end up breaking their software.
And some things in modern UI's are completely counterintuitive. Like the Chrome app for Android, the settings like "enable cookies" or "enable JavaScript" are filed under "content settings" whilst most people would expect it to be in "security". I can understand why they did this as cookies and JavaScript affect your content and they did this to not confuse less technical people. But security stuff is a functionality and the things I'd expect from content settings would be how the page is presented to me and what content is shown or hidden. Treating software like an experience rather than a tool makes things more confusing when less technical people have to deal with technical things.
All in all modern UI's provide very little customization and move stuff that was cemented in our brains for years and make them ten menus away and are designed around a "one for all" philosophy that only panders towards the devices they're broadening to and has a negative effect on users of the original hardware it was made for, which can easily be resolved by making separate interfaces for devices.