Crun can be/is used by podman/docker too so you could think that its essentially stripping docker down to its core of running oci containers and just taking the most lightweight and (fastest?) approach and embedding it directly into the executable.
I am sure that I might get somethings wrong, I usually do, so if I have an error, please do let me know as I don't wish to spread misinformation
This project on the other hand seems to be using bake https://github.com/losfair/bake which seems to take something like a docker container and I think essentially push it into something like .iso using squashfs The idea here is that they seem to embed firecracker itself into a single binary to actually just make it an executable that can run that microvm as it is.
And I think that bottlefire just seems to do something similar but they do it on their server side and seem to just provide you a binary to make things easy for you...
I am not sure with microvm's architecture but it seems that there are some additional features that you can get since they are VM, maybe you can get the ability of https://criu.org/Main_Page out of the box but this doesn't seem to be done right now but In my opinion is really very doable.
Another idea might be more security. I am going to refer to an article [1] which talks about flatpak but it has this line and I quote `containers are not sandboxed`, so I suppose that this can help blur the lines b/w containers by converting containers into microvm's and then leveraging vm's technology for sandboxing or some other mechanism rather easily as compared to docker (I see it use landlock which was new to me!)
article [1]: https://hanako.codeberg.page/