[pxc]

Here's the closest thing to an 'unwarning' that can exist, which is a tl;dr of the history:

The experimental label is a historical compromise because flakes, which was conceived by the original author of Nix (who is still a maintainer), was a major change merged to master without going through the whole Nix community RFC process (which was then in its infancy).

So it was added, but tucked away behind an 'experimental' designation and enablement flag. This proved controversial at the time and perhaps still is among old timers, but in the interim flakes have become enormously popular with new users (probably most people who picked up Nix in the past 3 years or so).

Third-party adoption of flakes has by now reached the point that the vast majority of big tools in the Nix ecosystem support it well. Some interfaces are still officially unstable but serious breakage is pretty rare. The flakes implementation in the codebase is very usable but still incomplete and still a bit buggy, and straightening out the kinks in the implementation as well as stabilizing the interfaces (both for Nix code and the CLI) is careful, patient work under way right now.

Hence the common recommendation to just use 'em unless you have deep Nix knowledge and a specific reason not to.