Very little. We at Working Title have managed to thrive just fine on what SDK is there, if developers are willing to work on the new tech stack. We’ve managed, on the CJ4, to implement a whole raft of detailed systems and features, include full and accurate working VNAV, RNAV with LPV, an entirely new flight plan system from scratch, FADEC, holds, missed approaches, correct custom fuel flow simulation, autopilot customization, etc.
Is every little feature there for every specific developer request? Perhaps not. But the limitations are not nearly so much on the simulator end as everyone keeps assuming. The sim can fully support something as complex as the PMDG 737NG, today, right now.
The real key, though, is embracing the new technologies.
-Matt