99,9% of the people already have to believe every other piece of code of bitcoin because they (me too) lack the skills to review it themselves.
This, in my opinion, is one of the biggest hurdles for bitcoin technology to overcome.
But they don't have to believe _me_, even if they can't review it themselves they can choose anyone else who has (or pay someone to, or learn...).
C++ is hard to read, much harder than C, but not impossible.
I'm not sure of the context for this comment, libsecp2561k is plain C.