This reminds me of the 5 year period from 2007 to 2012 when Apple made Safari for Windows. I got curious and used it for a few weeks, but moved back to Firefox for the extensions.
Chrome and Opera are technically blink but are essentially Webkit AFAIK (Google forked webkit and Opera followed). At least I haven't heard of any huge disparities between them.
Making browser cross-platform gives a bigger user base and instills confidence in developers to take a browser into consideration when designing websites, which in turn popularizes the browser as sites work properly.
Seems unlikely to ever render identically. At the very least I believe chrome uses the platform specific font shapers (coretext on mac, cleartype on win).
So in the end, a mac version of edgeium just adds yet another browser to the test matrix columns....