IME, the only real pain point with JJ is Git integration. I love it, it's great in almost everything it does. Formalizing all the DSLs and using them consistently throughout the config and CLI is exactly how filesets, revsets, and templating should be implemented. The focus on exposing concepts as interfaces (without "directly reflecting internals"[1] in the UI) is only as good as the concepts themselves, and in JJ's case, they are powerful. The problem is that they are too flexible, so Git interop relies heavily on convention and discipline... which is fatal. The fact that (almost?) all states of JJ are representable in Git makes it easy to mess up your remote, and then you need a very solid understanding of Git to fix it.
If you can give `jj` a try on a greenfield, single-developer project, you probably should. It feels exactly like Git felt compared to Subversion (when you focus on JJ's strengths and use it in favorable conditions - it is young, after all). Get acquainted with `jj` on its terms, and if things click for you, you'll be a bit better prepared for the bumpy road that is Git interop later on.