Port-forwarding on EdgeOS benefits from a simpler implementation and internally they track your WAN IP. VyOS doesn’t have this. So setting up DNAT with a moving WAN IP as the target isn’t natively doable. Not without some workarounds etc.
The move to a fixed IP meant this challenge went away, along with no longer needing the generously implemented feature to supply DHCP options.
Totally aligned with the items you note with Ubiquiti. UDP reordering issues have been going on for years, fixed in v.1.10.x and regressed in v2.x. Shows how reliant they are on their SoC provider that a move to a new Debian baseline brought this long-running bug back.
I had many issues with igmp-proxy, either not starting or crashing. Solved with a kludge to keep checking the service and restarting if needed every x minutes, but no real interest in the forum.