@Tobias Strauch even in the proprietary tools hierarchical STA is not frequently used. It is reserved for very large designs where flat is impossible. There is an accuracy loss which is unavoidable at the boundaries. Using an SDF doesn't sound like a good option for this flow. It would be better to run the design flat and let the delay calculator do its work. OpenSTA does not support SI or CCS yet. It is not signoff qualified. I recommend using the proprietary sign off tools when possible. If you can't be sure to add guard banding in the SDC to cover those inaccuracies.

Thanks @Tom Spyrou for you reply.