I think different people will have different answers. The biggest complaint is that this slows down builds, of course, but it also requires hacks to avoid re-including the same header twice when you have complex header hierarchies [1][2], bleed-trough of macros and pragmas, and of course you have to write forward declarations for everything you intend to publicly expose, as opposed to using a "public" keyword.

Honestly I dislike them because they're a redundant pain in the ass. I tried other solutions before (at one job I had 15 years ago we had automatic header code generation, ooof), and although modules are not that ready for primetime [3] (dependency resolution need to be done by an external program), I think they'll be the bees knees.

[1] https://en.wikipedia.org/wiki/Include_guard

[2] (some people will be quick to note that can be solved by pragmas)

[3] https://vector-of-bool.github.io/2019/01/27/modules-doa.html (but he wrote a follow up: https://vector-of-bool.github.io/2019/03/04/modules-doa-2.ht... )