Most criticism of the battery life of the Nexus 6 comes from users of another platform, who's larger (for them) flagship does have substantially longer battery life than the Nexus 6 in most usage situations.
Of course, those users tend to be or pretend to be unaware of the fact that they are making an "apples-to-oranges" comparison where the one (we'll call it the 'apple') has longer battery life, but largely due to the sacrifices they make in accepting a smaller display with a fraction of the resolution of the other (we'll call it the 'orange').
They also tend to ignore the fact that for nearly every user, either device will easily makes it through a full day of heavy use, for which battery life that extends it beyond that duration is essentially irrelevant unless it successfully spans two days - a feat that neither does.