When is using Flatpak not advised? Or should we all switch to only using Flatpaks?

I know Flatpaks are sandboxed which can be useful, and can also help avoid dependency hell (at the expense of a slightly larger package size). But are there times where using a system package might be better? I've heard some people say Flatpak is good for GUI applications only, but is there any credibility to that claim? Would an application like Steam for example perform better as a system package or Flatpak? (A popular GUI app I've heard people claim runs better as system package instead of Flatpak)