Also worth noting https://lwn.net/Articles/1097760/ - Gentoo has given up on building Chromium from source, because upstream "do not test their builds with any of their allegedly 'optional' features disabled" and "[i]n the case of Chromium, keeping all the toggle switches working (despite chronic neglect from upstream) is a monumental, nigh-insurmountable task." One maintainer said he could not possibly test all the combinations of USE flags in the Gentoo build, since his laptop took 15-16 hours to compile Chromium (I presume for just one build, which would have to be repeated 2^N times to test all possible flag combinations).
Much more in the LWN article, I'm not going to quote everything.
Much more in the LWN article, I'm not going to quote everything.