Comment 13 for bug 1729576

While snapd can read the home directory from the gecos system it is not the technical difficulty preventing the use of arbitrary home directory. This feature is currently unsupported and is listed as such on https://forum.snapcraft.io/t/limitations-in-snapd/9718

I'm sorry for the inconvenience but we had to place those limitations on the user's data location for now. As a workaround you can always mount the desired volume instead of using symbolic links. You can store /home/user anywhere you want as long as you use the mount system to do that. The same goes for /root.