1. Reflog is very ref specific that is it tracks head. JJ has op and evo logs that do that + a bunch more they track everything about the repo every possible action. It's hard to explain with going into technical depth but you can read up on it.

2. Yeah submodules aren't supported but since you can use jj in a git repo you can commit git stuff via git like submodules and then enjoy the world of jj. Since submodule interactions should be rare imho. I use it for a bunch of projects with submodules.