One aspect is that the server side is not free software. So only Canonical can reasonably host a repo of snaps.

That was enough for me not to look into the technical merits of the clientside implementation. The few experiences I had with snap was that programs were starting slowly (Firefox) and e.g. GPU acceleration did not work (vlc). Not sure how good the sandboxing is. But I run my Firefox sandboxed by firejail and the overhead can not be felt in daily usage.