This hate comes mostly from Linux communities like here and on Reddit. When you see actual numbers, both are widely used for production use. They have lots of active users as reported in their respective blogs and websites.

That said, it is aware that both had problems. Most hate towards Flatpaks that I can see is from purists that prefer their distro shipping their packages with dynamic dependencies and uprated by their package manager. Also there is complains with outdated runtimes and stuff like how sandboxing works.

Snaps has all problems than before with some extras. When they were released, because of compression, they were painfully slowly to open and they affected boot time. Nowadays this is mostly gone, but they still keep a proprietary store, inability to have multiple repositories (stores) and they don't respect your home directory structure by placing a "snap" folder in your home.

Personally I use both and I'm happy with them. The proprietary store stuff does not bother me because I'm already trusting canonical binaries by using Ubuntu and they are easy to use and be productive with them.