>Even spidermonkey, the javascript engine, is never reused as v8.

GNOME and Cinnamon desktops use it, MongoDB uses it, polkit, etc. It's obviously not embedded nearly as much as V8 but it does have a few.

But aren’t Brave, Edge and Opera based on Chromium instead of only Blink? Am I missing something?

Blink is a part of Chromium, but not really a decoupled part. Having blink without the rest of Chromium isn't really possible at this time. Some details at https://www.chromium.org/blink https://www.chromium.org/developers/content-module

I think the point is gecko is to blink, as firefox is to chromium. Sure, there are various browsers based on chromium out there, but they're not embedding blink. They're reskins of chromium with additional changes. How much more difficult is that compared to what goes into making iceweasel from firefox, I couldn't tell you.

But the "embedding gecko is hard, embedding chromium is easy" explanation certainly seems to be wrong, in terminology at least.