Will a rain cover keep my pack dry in heavy or all-day rain?

No cover is fully waterproof in sustained heavy rain. r/backpacking and r/AppalachianTrail threads repeatedly note that water eventually finds its way in through the back panel (which the cover doesn't protect), seams, and the cinch opening. For multi-hour downpours, the consensus is to pair a cover with a waterproof liner or dry sacks for anything that must stay dry, like a sleeping bag or electronics.