Microsoft Teams. I am forced to use it (work) and it is missing really basic features that messenger software had in the 1990s like Push-To-Talk, real multi-window (even with the recent "pop-out" functionality), and its UI is all the worst modern trends. You cannot extend it or fix these issues (e.g. plugins, custom CSS styles, etc). Plus it is buggy, I keep not getting calls/messages/etc, and every time my computer…
Audio is frequently bad too when people speak alternatingly. The new speaker has 2-3 seconds of muffled audio until they're clear again. Especially annoying when speakers rapidly change and all you get is semi muffled audio all the way.
Also the inability to respond to a specific message on desktop has baffled me. It seems like a basic functionality in chat software these days but teams only decided to give it to mobile? Just... Why?
And many emoticons have been renamed or simply removed. Why reinvent the wheel ?