I'm not an expert, but my understanding of the Global Shortcuts portal is that it's very much designed for the push-to-talk use case where an app is not focused but still receives button events for exactly the keys its interested in and no other keys: I think this would cause problems if an app requested every key (e.g. if the request was approved then no keys would work in every other app)
It'll be interesting to see how the remaining compatibility/accessibility issues are tackled, either in portals or in wayland protocols
I did actually do this already, separate from working on this issue, but can confirm the intermittent problems with the combination of wpa_supplicant and systemd-networkd