It's the elimination of window borders. Aside from not being able to differentiate one window from another similarly colored window in the background, it's nearly impossible to click and hold on anything along the edge to resize the window.
It's the overloading of the title bar with so much shit like search boxes and extraneous buttons that a user has almost no place to grip to move the window.
It's the way that tabbing between text boxes either doesn't behave the way you'd expect, or doesn't work at all.
It's all the tooltips that interrupt and litter the interface and, at times, block out things that you are looking at. And 95% of the time, the information provided in these tooltips are redundant or useless.
It's amazing how much damage these cargo-cult UI/UX morons have done in the past ten years. They threw out several decades of usability pioneered by real HID experts for something that looks pretty but doesn't fucking work for a lot of people.
Applications like Postman, Teams (and pretty much all of MSFT's applications these days), Chrome, and Insomnia should be case studies on how to not design user interfaces. They are about as bad as desktop software gets.
The biggest sin is that this would be a non-issue if these things were configurable at the windowing system level and could not be overriden by app developers. But the trend has gone in the opposite direction; instead of providing more configurability, Windows and Gnome/GTK are actually taking away options that have existed before.