Officially supported doesnt mean its more stable.
Never implied that anyways. Official merely ensures that the amount of trusted parties can be minimized.
Bubblewrap is not insecure.
Bubblewrap, when properly applied is indeed excellent; perhaps the best utility to sandbox applications on Linux. I'm thankful that flatpaks makes use of bubblewrap, namespaces and seccomp to offer relatively safe/secure apps/binaries, I'm unaware of any other '(universal) package manager' within the Linux-space that offers similar feats in that regard. Unfortunately, Chromium-based browsers just happen to have an even stronger sandbox -if properly configured- than flatpaks are currently capable of.
Would you mind elaborating? First time hearing this and a quick search didn't resolve it.