Pretty sure Micropython is also not compatible with the UF2loader. The only viable option of MicroPython (or CircuitPython) for the loader that I’m aware of is the PicoWare builds. While they aren’t the same as the official MicroPython builds, the code itself is mostly the same but with extra platform framework stuff added.
Seems like folks that are into PicoMite/WebMite don’t generally want to use other stuff anyway, and people using MicroPython haven’t been very active on the fourms. Judging from the sheer number of different MicroPython ports to the PicoCalc on github (note: not a complete list!) it seems like most MicroPython users end up building or customizing their own version and then disappearing into the ether, perhaps after sharing a repo on github. And so far it seems no one has had an interest in getting them to work with the UF2loader, which implies that folks who use them don’t use anything else on their device. Also, code or related discussions from any active projects has almost never been shared on the forums, except for LofiFren who occasionally posts about updates to their port.
MMBasic code/projects do seem to be shared here quite often though.
Pretty much everything else is easily swappable using the loader though. Sounds like ZeptoForth has only experimental support for the loader now (which will become official at some point, no doubt). A lot of other projects were built out of, or on top of the PicoCalc Text Starter project, which is compatible with the loader and so things based on it generally are too.
Since practically everything supporting the PicoCalc is community driven, there’s no real organization across projects, so support is kind of scattered and that’s why not everything works with the UF2loader. Projects support it only if the devs on those (often personal) projects choose to put in whatever effort is needed to support it.
Personally, I’d love to see everything supported in the loader, since I find it incredibly useful to swap between things. An old, experimental version of PicoMite does actually work on the loader though, but if you want/need to use the latest PicoMite version, as most people seem to, you’ll have to wipe and flash only it to your device.