Bernardo wasn't ready in 17/18 season, he was learning the system after coming from another country. After that he became the best player last season.
Bernardo and Sterling are better for Pep's system for two reasons. First, we play most of the time against deep sitting teams and Sane can't operate in tight spaces as well as Raz and Bernardo. Second, Raz and Bernardo can keep possession better than Sane which is highly important for Pep for it makes us less vulnearble to counter attacks. We conceded only 4 goals in the last 14 games with Sane being mostly on the bench. Coincidence? Probably not. There are stats showing that Raz and Bernardo keep the ball much better than Sane and thus give us additional stability in our defensive organisation. When people think of Sane, they think mostly of his amaizing runs and not of his cheap mistakes at controlling the ball and half-arsed attempts to win it back which make Pep mad.