I tested a release upgrade from Ubuntu 16.04 to Ubuntu 18.04 today using the dist-upgrader from -proposed. Because its not possible to use do-release-upgrade -d and -p, I manually downloaded and extracted the dist-upgrader tarball and ran 'sudo ./bionic --frontend DistUpgradeViewGtk3'. This was on a virtual machine with 4 GB of memory. The process of installing snaps was displayed on the screen during the upgrade process, however the dist-upgrader dialog did freeze a couple of times perhaps because of the low memory. Regardless this is an improvement over the original situation and may be good enough.
I tested a release upgrade from Ubuntu 16.04 to Ubuntu 18.04 today using the dist-upgrader from -proposed. Because its not possible to use do-release-upgrade -d and -p, I manually downloaded and extracted the dist-upgrader tarball and ran 'sudo ./bionic --frontend DistUpgradeView Gtk3'. This was on a virtual machine with 4 GB of memory. The process of installing snaps was displayed on the screen during the upgrade process, however the dist-upgrader dialog did freeze a couple of times perhaps because of the low memory. Regardless this is an improvement over the original situation and may be good enough.