As another data point: I used Fedora Silverblue for a couple years, switched to Kinoite (Silverblue - Gnome + KDE) without reinstalling a couple months ago, and

As another data point: I used Fedora Silverblue for a couple years, switched to Kinoite (Silverblue - Gnome + KDE) without reinstalling a couple months ago, and updated to 40 last week, and everything seems to be working fine. While I can't vouch for Gnome, the only KDE migration bug I noticed was that the update slightly rearranged the items on one of my custom panels and reset my desktop wallpaper (the latter may not be a bug). As for development/hackability: you need to understand ostree, rpm-ostree, and flatpak fundamentals, not be afraid to layer packages where containerization is impossible or inconvenient, and accept that cookie-cutter configuration recipes intended for other distros aren't likely to work. I'd also recommend distrobox over the bundled toolbox for managing distro environment containers, because useful additional features. With all that said, I've been using Silverblue+Kinoite daily as a development (Linux + Windows in libvirt-managed VM) system for over a year without no regrets whatsoever, and only trivial complications (e.g., running a trivial shell script to rewrite JetBrains Toolbox-created .desktop files so JetBrains IDEs run in my preferred distrobox container when launched via GUI). As a shout-out to another Fedora immutable distro, I've also switched to Fedora CoreOS for new home office network service deployments and this has been similarly pleasant and hassle-free.