I use them for some things and I think they are fine. Mostly apps that are kinda messy and I want to keep them and their atrocious dependency tree away from my base system. I also like to use them for proprietary apps or apps where I actually want to use the sandbox. Other than that I prefer native packages 99% of the time.

Flatpak is slower to update than pacman, the cli interface just doesn't feel good to use. There is the weird naming, no real way to get a dependency tree, can't hide those annoying eol messages even for apps that I specifically don't want to update. Another thing is that not every app was made to run in a sandbox or it is just more difficult to use sometimes. A lot of people tend to cite ide's, but in my case I was having issues with the steam flatpak. Running games with steam was fine, but anytime I wanted to hook up something third party eg: mods, cheat engine, etc. Doing so in the flatpak either required some tinkering around the sandbox or straight up didn't work.

I feel like that last sentence sums up the whole experience. If you just need to point and click and have it work. Flatpak does that amazingly. If you need any kind of integration with other things, expect problems.

Edit: just wanted to add that, the whole point and click and work is fine for 99% of people which is why I and many others choose to use it.