If you're talking not stable as in it's rolling release, then yes that's true although it isn't unique to Guix.

But if you mean leaves your system vulnerable to breakages, large unwanted changes, system entropy, or inconsistencies, then surely Guix (and Nix) are more stable than virtually every other distro (and maybe even MacOS or Windows)?

I'm also not sure I would say Guix is particularly unpolished compared to other distros. The project is 10 years old at this point, has a very active (albeit not huge) community, and has some of the clearest/most extensive documentation I've read for a distro. Perhaps 5-ish years ago it had some rough edges, but I'd say now they're quite comparable to other mainstream distros.

As someone who has been using Guix as my primary OS for the past ~2 years on a <5 year old laptop (with a proprietary wifi card) for programming, media, web stuff, and light games, I'd say it's a great experience. Not that the learning curve is not large or that community resources couldn't be better, but if you're already using GNU/Linux I don't think it will feel any less stable/polished.