It took me a lo(oooooo)ng time to finally test network-manager 1.10/stable with our gadget stack, so apologies for the belated reply. I’ve since migrated our application to use NetworkManager’s D-Bus API instead of nmcli, which works fine with both the default and 1.10 tracks of network-manager. So we don’t need to advise existing gadget users to switch to the 1.10 track before we roll out the core18-based update for our main snap.
I was hoping the switch to NM 1.10 might provide more insight for some WiFi issues we’re experiencing on the Pi 3B+ (fails to join 802.11ac networks reliably even though the 3B+ hardware is capable of this), but those seem to stem from bugs in the Broadcom drivers. Did I already mention that low-level WiFi debugging isn’t my field of expertise 
But as far as I have tested it, the 1.10/stable channel works just fine. Thanks @abeato & Co!