The main problem was that, as far as I could tell, the window manager couldn't really preempt the windows and force them to show up in a certain way. Instead it basically asked them "pretty please, could you show up at this location with these dimensions?". As a result frozen apps (such as graphical apps that triggered a breakpoint in code or are simply misbehaving) or apps running as admin would refuse to obey and stay stuck. IIRC one workaround was to run the window manager with admin privilege but even that didn't help with frozen apps I think. Besides I wasn't really comfortable running such an application with administrator privileges.
Of course that was a long time ago and it was a non-MS project, hopefully this one works better. I really can't see myself working without a tiling WM anymore, "conventional" desktops feel terribly clunky and inefficient to me.