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omitting sites that are not mobile-friendly.
The problem G have is that they can't identify a mobile-friendly site with any reliability at the moment.I think Google does a pretty good job with identifying mobile-friendly sites. Maybe their criteria doesn't please everyone, but IMO their guidelines/suggestions are usually very helpful in getting mobile sites to display & perform better for most users.
There are just too many spammers trying to appear to be mobile friendly who really aren't.Not just "spammers." Go back to when Google released it's mobile-friendly check tool. The buzz around here was mainly focused on how to game the tool and get your site to pass without actually having to rebuild your site to be mobile-friendly.
Probably just to save writing "Mobile-friendly"
.. it would mean different SERPs for desktop and for mobile
.. that was a joke aimed at the Google's SERP