There is a a debate about whether cellulose or fiberglass is better. There is no debate that insulation must be installed properly. The cellulose industry highlights fiberglass as an inferior material due to gaps and compaction. A conscientious installer, whether cellulose or fiberglass will pay attention to those issues.
The following illustration shows a proper technique to installing fiberglass bats. To eliminate gaps, 94″ material in a standard 8 foot wall. That allows a small tuck at the sill plates (A). The material (including vapor barrier) is tucked behind pipes and drains (B). This allows the interior warmth to keep pipes from freezing. A polyurethane foam is very effective to fill small spaces between studs (C), windows and doors. Tuck fiberglass and vapor barrier is behind electrical outlets to prevent air leaks. Since the fiberglass is compressed in this area, r-values are compromised. Surround the outlets with polyurethane foam (D) to bring the r-values back up and seal the plugs and wires from air leaks. Use foam to address the sill plates, corners, and other minor cracks (E).