In practice, for more "modern" projects, it works fine: they tend to have good hygiene anyway. For Rust projects, for example, you .gitignore the target directory, and you're good for 99% of projects (and cargo new already generates said ignore.)
For older projects that leave temporary files everywhere, or for workflows that may generate intermediate files (like coredumps are often generated in the cwd and not somewhere easily ignored), it can be a pain.
However, in the next release, a feature related to this has landed: you can add a configuration, `snapshot.auto-track="none()"`, to turn off automatically tracking files, and then `jj track` individual paths (with globs, of course), if you prefer that workflow.
It basically works fine, but there are some annoying edge cases that even I, a jj developer, have run into. Probably the most "common" one (and it's not that common, just moreso than the rest) when you switch branches with different `.gitignore` files, you may stop ignoring some junk from the other branch when you switch. Kind of annoying.
But it's overall pretty marginal in projects with good ignore hygiene, and there are solutions to that problem too (to varying degrees of ease.)
It feels like you'd end up with a bunch of junk in your repo.