It's Canonical's (the company that created/updates/supports Ubuntu) package format. There are a few problems.

They can only be hosted on proprietary Canonical servers. That sort of flies in the face of one of the "free" aspects of Linux. Canonical is also sort of fostering a reputation of abandoning/massively changing something core in Ubuntu every couple major releases, which has made some wary of depending on snaps, since if Canonical decides to stop hosting them, anyone dependent on them is kinda screwed. Snaps can also chew up disk space if you're not careful. I don't think that's necessarily unique to snaps, but in my experience that issue has been worse with snaps than with comparable alternatives like flatpaks.