How bad is it that the battery adhesive failed after phone reassembly?

Well usually internal things are made to fit securely and safely right along one another or close enough. Usually a battery in cellphones is the biggest and heaviest internal component. Dismounted battery could eventually with enough force dislodge other internal parts or damage itself. And disconnect or damage wiring.Its definitely not good to have your main source of power free falling with every motion. This could cause positive and negatives to eventually cross and and fry delicate parts or your phone completely.