I agree, in particular by the way unity builds can break code that relies on internal linkage to avoid conflicts between symbols. But it's nevertheless a technique that's supported by cmake and can be enabled by flipping a flag.
Chromium eventually dropped support for such builds. At Google the developers have access to a compilation farm that compiles within like 5 minutes. With such farm unity builds makes things slower as they decrease parallelism. So Google decided not to support them not to deal with very occasional compilation breakage.
I lost count of the number of times that I had to deal with cyclic dependencies accidentally introduced by someone when they started to add #include without any criteria.