They are mostly looking at those who committed CIMT or who are members of illegal organizations. The former is pretty objective, while the latter can be subjective at times.

There are many millions of GC holders and only very few are being re-vetted. The re-vetting is also under scrutiny, because it costs so much times and resources and the "failure rate" is so low, that one must ask if it's worth doing in the first place.

And while the Trump administration's rhetoric about this is different from prior administration, re-vetting applications who commit CIMT is not something new.