It's clear to me that #flathub is the go-to vendor for Flatpak package distribution and as such, it sort of becomes a vendor lock-in situation... but #Flatpak supports third party repos OOB and #snap doesn't.
Flathub has also been held to the fire and it lead to a change in policy, thanks to the likes of @BrodieOnLinux shining a light on certain issues, like flathub enforcing #gnome like requirements and spiting #QT apps, which has since been changed to be more inclusive.