The breaking/notable changes are collected in the respective release notes section of the manual as they happen. https://github.com/NixOS/nixpkgs/blob/9ac60d31c0fa0bb2bbd7d166c4eaf19e9ba8db35/nixos/doc/manual/release-notes/rl-2311.section.md for 23.11 for example. The current master branch will be branched off into the 23.11 release branch, so in the past few months, that’s where breaking change notes on the unstable channel were collected.
There’s no specific “good” time to switch to unstable though as these notes are always there and breaking changes come in all the time. The exception might be this time of the year since breaking changes are now restricted but it’s honestly not a large difference.
Additionally, there is Breaking changes announcement for unstable where some (not all) breaking changes or changes that need more thorough testing only possible with a wide amount of use-cases or changes that might be less visible (such as core library updates) are posted.
I must say that, generally, the unstable channel is pretty stable. While they do happen from time to time, breaking bugs are rare. Additionally, it’s not like you’re stuck when there is one since you can simply rollback the Nixpkgs update (it’s still NixOS afterall!).
The most “critical” thing you might need to do from time to time is change up how you configure certain things because the interface was changed. You get an eval error for this telling you what changed and you can usually trivially change it.
This too is rather rare though. Happens 1-2 times per year for me perhaps? In all likelyhood, an upgrade from stable to unstable can happen without any changes to the config necessary and will work just like before.