The lack of version is the problem. Syntax has changed over time, so when someone finds or has an older compose file, there is no hint it won’t work with the current version of docker-compose until you get errors and no graceful way to handle it.
Compose doesn’t have a versioned standard, it did for a bit iirc, which also means you can’t always just grab a compose file and know it will always just work.
Most self hosted works fine with giant all in one containers, even for complex apps, it’s when you need to scale you usually hit problems with an all in one container approach and have to change.
By syncing data, it isn’t all data, just that it requires non-local resources, ie cloud/API, to function. You do have to look at each integration to see what it is doing, I would expect a Spotify integration is just hitting the Spotify API and maybe can interact with local devices that Spotify can stream to (ie a Chromecast)
Modern UEFI in boxes has http boot options generally, and ipxe has supported http boot a long time. though I still get the grub2 bootloader bits over tftp, then http for kernel and initrd.