To be fair though: that's a gentoo fault. They want to be able to be as customizable as possible, which is fine, but if you support all flag-combinations, you are going to either test them all, or forego testing them. It's also a bit awkward that they don't mention this with regards to the linux kernel. Are the use flags that limited to begin with?
Then those chromium flags should not exist.
I'm betting the Linux kernel actually tests their config combinations. The complaint is that Chromium doesn't test turning off their allegedly-optional features, so when one of those features turns out not to be as optional as it claims to be, it's Gentoo maintainers who bear the brunt of discovering that (or users end up finding it out if it's not tested).
I can't tell you what Gentoo does with the Linux kernel in terms of USE flags; it's been over a decade since I last tried Gentoo and I don't want to spend the time looking it up since I doubt anyone truly cares. (If someone does care enough and looks it up, I care just enough to be curious to know what you find).