It’s as close to a “universal packaging system” as can get now.
There was a lot of talk back in time, when Ubuntu decided to forcefully shove snaps onto users. The thing is, Ubuntu could have embraced flatpaks like many other distros but it chose snaps which is not ideal for people who like an OS whose primary goals revolve around freedom and privacy. You see, it is the proprietary nature of snaps that gets them this hate.
Appimage and other packaging methods don’t get this hate because they are open source and users have a “choice”. What we are seeing against snaps is the result of forcing people to a choice, ofcourse the people in question are linux users - people who are famous about taking freedom of choice seriously. Yes, you can get ride of snaps on Ubuntu but you can get rid of lot of ads and stuff on windows with a lot of tinkering too - I think you see the point.
Many people tend to have a preference for flatpaks because they do basically what snaps do but better and ofcourse flatpaks fit into the “freedom and privacy” spirit of linux.