In a shared pool, reputation is a function of every sender using it, not any one sender alone. This benefits low-volume senders who couldn't independently establish strong reputation, but exposes every tenant to any other tenant's poor practices.
Pool operators typically monitor and remove senders whose complaint or bounce behavior threatens the pool, but that protection is reactive; a sudden bad send from one tenant can still cause a temporary reputation dip felt by everyone else on the pool.
The trade-off against a dedicated IP is largely about volume and control: enough consistent volume to sustain independent reputation, and enough need for isolation from other tenants' behavior, both favor moving to a dedicated IP.