Linux networking has been a solved problem since just about forever, and all this instability will do is cause people to abandon Ubuntu in favor of something less fickle.
It would be more useful for the various major distros to pitch in for easy, simple, congruent CLI, UI, declarative, and API configuration management for common advanced configuration.
Creating artificial lock-in that doesn't work is a double-barrel footgun blast.
The Ubuntu people are like the SystemD people: they just want to reinvent things from scratch and shove it down everyone's throats without any consideration.
This, I really don't understand that they want to create a diverging solution for something that already works really well. We don't need 15 different solutions to configure networking.
Imagine looking for a Java solution and the only response you find is in Kotlin... fucking annoying as I have to learn Kotlin to convert it to Java...
The same thing with network configuration, you're looking for something that works on Debian and you only find a Ubuntu solution....
What they have done is taken the chaos of networking scattered across 1/2 dozen (or more) configuration files and commands and centralizes so the chaos is in one place. They've succeeded in simplifying some parts and I have faith they will make network configuration much simpler in the future.
You only think ifupdown-ng, nmcli/nmtui work well because you don't remember how hard it was to learn and developed scar tissue from using them. To my scar tissue, Debian/Ubuntu is well organized and tools like netplan clicked very fast and make network configuration much easier.
Guessing, but I think it probably has more to do with the various tools and frontends (GUI, cloud-init, etc.) designed to manage the configurations than it has to do with the capabilities of what the configuration system will support. I have been bitten by some of this too, but the various script hooks are still there, and I've been able to support everything else via those mechanisms.
They're trying for lock-in and failing.
Linux networking has been a solved problem since just about forever, and all this instability will do is cause people to abandon Ubuntu in favor of something less fickle.