Like josh said, I don't think that's necessarily much to do with the OS itself.
The messaging app has been poo since the start on W10, with texts going missing or no notifications appearing, it being slow and laggy as shit etc. The latest update that came a day or two after the latest build has made it at least a lot quicker for me, but I don't doubt that it's still buggy as fuck.
Outlook has been pretty ok for me for a while, though I did have to re-pin the tiles after the latest update because they were no longer updating. However, they syncing now seems to be much more reliable than before (emails are arriving immediately now, as you should expect).
I do think there's still some work to do. Battery life is worse for me for some reason since the latest build and I also think that there's still all sorts of problems with the updating process itself where a lot of bugs and problems come from and why hard resets are still necessary.
I've given up hope that they'll fix the visual inconsistencies in time for launch since there's still a lot of under-the-hood work to be done. But I am pretty hopeful that it will run smoothly and relatively bug free on the new phones when they are released.