i haven't upgraded my daily driver machine yet, but i did play around with it on an old machine and it wasn't stable enough for me to want to migrate yet. as per your question, it's similar i guess, but kde plasma is a lot more mature and built on rust.

I've been meaning to check it out. I keep specialisations of my system config for all the major DE's, but I haven't added one for Cosmic yet. (or Niri)