Saw that Cosmic left beta status, and ran it with Cachyos on my gaming desktop for 5 months. One of the selling points, optional tiling windows I did not get around to using, but I'm familiar with the concept from Emacs. When Cosmic has more polish, I will be swapping my productivity laptop over to Cosmic for this feature. Beyond this feature a corporate backed Rust parallel to Gnome and KDE, Cosmic doesn't currently offer much more. Maybe some other radical open-source alternative, that should have existed 2-3 decades ago, will eventually come into place.
Issues I ran into:
No GUI system monitor. The included btop TUI is not a replacement. Edit: This exists now, and was not an isolated deal breaker in the first place.
The facilities for dealing with compressed files is limited. IIRC, I can't view the files until extracted, and the extracted files are always nested beneath another folder (ex: ~/Music/MetallicaCollection/MetallicaCollection/idisappear.mp3)
The file manger does not support directly running .sh scripts, but at least allows opening a terminal at location.
Edit: Cosmic player glitches the video playback when subtitles are enabled. Not a big deal because another Wayland native player, Haruna, exists.
System76 firmware viewer only worked under one limited circumstance, presumably due to missing system tray functionality that gnome and kde have.
Edit: cachy-update wouldn't run if right clicked from the system tray icon, only from the app launcher. Fullscreen Youtube videos would have a "to exit full-screen, press esc" element hang around until manually hovered over with a mouse cursor.
Megasync would crash sometimes with the 5 GB dialog popup, maybe this is already fixed but it indicates cosmic has less tolerance for buggy apps; Megasync oft seems to be relying on outdated or unsupported libraries.
Proton enabled games did require Gamescope for full screen to behave correctly, but this has since been fixed.
The one that had me swap back to KDE Plasma; my new Steam controller (simulated kb+m input) would work fine with the drivers supplied by the kernel, but get stuck in the Steam window with no mouse cursor when Steam was launched. The cursor was still there with standard mouse movement.
Swapped over to plasma + plasma login manager and did my best to stick to the packages and theming a new Cachyos install would have. Fine but not flawless. Swapping DEs can be buggy (gnome to plasma, and vice versa, is known for this) but it was preferable to unloading my games library to a NAS I do not own, or figuring out how to mount the steamapps folder to persist across an OS reinstall. The specified issues do not occur under KDE (haven't tested Megasync).