I know that many X-Wing players believe skill is the biggest factor in play, all things being equal... That said, all things are not equal. Imperial pilots and ships have more hard counters than Rebels. Scum is in a low choice situation currently, so they notice even more than imperials, perhaps, that while squad building many options are untenable.
I think the nature of shields makes many rebel ships inherently safer with regards to hard counters. Everyone from Turr to Mauler to Rhymer etc. are up against the possibility of a hard counter before they hit the table. There is no insulation versus an uphill battle. Squad building for imperials requires more consideration for opponent's build.
Rebels can double dip in squad building. They have the opportunity to build to the meta and experience native resilience versus the bad matchup.

