I think XARs are much simpler. snapcraft (and AppImage and other tools) seem to be much more invasive to the development flow and more opinionated about what the contents of the image look like (directory structures, yaml/xml/json files for metadata, etc).

XAR is much simpler. All it needs is a shell script (or raw executable) to run inside the XAR. Everything else is up to the bootstrapping. Note this isn't always a good thing -- opinionated software has its place. For what we use XAR for, though, having it "just work" with standard Python tooling without expecting open source libraries or modules or build tools behave differently is a strong plus.