I tried using KOrganize which had KMail and some other stuff integrated together and ended up feeling like it was a gigantic, archaic codebase just hanging on by a thread. It struggled a lot with Gmail and several times I deleted my whole mail profile to try to fix some strange bug.
If I recall, what did me in was that it would stop sending emails after running for a while. The fix had something to do with restarting Akonadi. It was really disappointing, because I love a good UI/Plasma integration.