Modules support was added recently, and I don't think most libraries or cmake support it yet, and I don't really see tutorial about good practices for modules, especially when it comes down to speeding up compilation.

Also modules do not really speed up compilation that much, apparently, or I have not seen benchmarks, maybe because modules are not well supported yet?

Modules are great in theory, but I am not sure they are usable in 100% of cases, especially with all the existing code that is out there?