As a Guix user/occasional packager who has run into broken packages a few times (including Python ones)... I do wish it had a dependency resolver that could at least be run outside the build/install process, e.g. when updating libraries.

If I'm updating a library that has a bunch of dependent packages, it's hard to know whether or not something will break downstream. Sometimes this is unavoidable and you really do need to just test everything thoroughly, but sometimes the dependent packages have more knowledge of what library versions they need... and Guix doesn't seem to be aware of this.