Most importantly, submodules are not fully supported, which are used by almost every open source project at least in the space I work in (embedded). So you can'

Most importantly, submodules are not fully supported, which are used by almost every open source project at least in the space I work in (embedded). So you can't use jj to easily contribute back to those project. It can be done but you always have to be cognizant of whether a submodule has changed between two branches or when you sync, since they don't update automatically the way they do with git.