Is flat-pack furniture always worse than assembled?

No — flat-pack is a shipping method, not a quality tier. Plenty of flat-pack pieces use solid drawer boxes, metal slides, and decent engineered boards; plenty of "assembled" pieces are stapled particle board. What predicts lifespan is the material grade (plywood beats bare particle board at stress points), the fasteners (cam-and-bolt beats glue and staples), and whether replacement hardware exists. Flat-pack does shift assembly risk to you: torque the fittings properly and anchor tall pieces to the wall.