Very dense deployments of small cells are one of the key enablers to tackle the ever-growing demand on mobile bandwidth. In such deployments, centralization of RAN functions on cloud resources is envisioned to overcome severe inter-cell interference and to keep costs acceptable. However, RAN backhaul constraints need to be considered when designing the functional split between RAN front-ends and centralized equipment. In this paper we analyze constraints and outline applications of flexible RAN centralization.