A colleague of mine was also looking at Ubuntu Core for an IoT project recently, but Ubuntu wanted $15k/y to run a private, branded Snap store - erm.. no.
If they really want snaps to succeed, there should be an open source snap store protocol, and 3rd parties should be allowed to run their own stores, just like you can add 3rd part apt repos, for example.
We decided on Photon OS, BTW. It's tiny, and perfect for use as a Docker host.