Technical stuff aside, a major reason why Flatpak does not get the hate Snap does is because users don't resent it.

There are 2 things people hate: the way things are, and change.

Ubuntu put Snaps on users desktops without asking if they wanted them. They just did it, then closed off ways to opt-out/avoid them (Cf. Firefox).

Forced change fosters resentment.

No distro made Flatpaks default/required/opt-out. Users had time to come to the tech and adopt it on their own terms/needs.

Just IMO.

@omgubuntu I think it also have some echoes of Canonical wanting snap to stick around badly, so it feels forced a bit? Community around flatpak - and it has tons of controversies for sure - have just grown more organically, and it responds to challenges better. I haven't invested time to understand snap, so I have no reflection point against it.
@peteriskrisjanis Organic is a great descriptor. There's been very little organic about Snaps, even the big-name software didn't get there organically (the stories are out there of Canonical cold-calling developers to pressure them into adopting Snaps, and in many cases, doing the work for them).