I don't know how much of a rewrite their Cosmic Desktop is meant to be, but Iced isn't ready for anything but small, simple utilities. Known issues include a lack of partial rendering and thus high battery usage, no support for touchpad gestures and a lack of accessibility support. Beyond that you have the rough edges and countless paper cuts that come with an unpolished and unproven toolkit. People really seem to underestimate the amount of work that goes in to creating a "proper" desktop GUI toolkit. You basically need to build a webbrowser minus the JS engine. Even Microsoft has given up and basically uses WebViews wherever possible.