The paper discusses the main challenges in the grid integration of Battery Energy Storage Systems together with renewable power plants when Hybrid Control at Point of Connection and Grid Forming Control at PCS level is utilized. The real cases show that top challenges in integration of BESS into power systems are related to Grid Code compliance of Hybrid plants at POC of the relevant power system operator due to split of control functionalities between Master Plant Control, Follower controllers and Power Electronic converters with variety of control approaches. In order to control Renewable plant optimally the special approach needs to be applied allowing Grid forming capabilities are fully present at the POC. The modern Hybrid Plants are expected to provide GFM functionalities, ability to operate in Islanded mode and to provide system restoration support at the Point of Connection. Other challenges are related to realistic representation of the all-hybrid plant components and covering detailed phenomenon such as Latency by EMT, RMS simulations models of the entire plant. One of the main difficulties, especially when utilizing desktop computer simulations, is availability and interoperability of plant sub-components simulation models when several different OEMs are involved. Another issue is the lack of standardization of the entire plant model validation methodologies. The paper presents the approach where as a solution for components model validations in real time and entire plant control verification are the real-time simulations utilized.