It's slower to start than Paint (on this laptop at least), and I can't see any way to import an image using a scanner using it (one of my main use cases for Paint).
And why on earth does the menu icon in Paint 3D look like a folder icon?
Use Paint.net (fast load) or The Gimp (works very fine with scaners, but don't forget to enable single window mode)
I've used both, and like Paint.NET for more complex work, but both have too much UI clutter for the simple tasks I mentioned...
If MS can do something worse that MS Paint, is MS Paint 3d.
Paint 3D is actually a decent app. I think people are going to hate it by default though.