I guess R12 will be the most stable release ever, since
1. there are no new functionalities in it for HCM
2. the technology stack is simplified
(1) It seems all new functionalites have been released already on previous 11i family packs and RUPs. I only found some changes on the new financial legal entities, operating units and business groups (based on hr security profiles!) and checklist Self Service functionality.
(2) No 9 database anymore. Only a few Oracle Homes. Fusion middleware.