We have a key plugin that is written in c++ for performance and availability of e.g. custom grips.
Also, limitations in the RTree as discussed here [RhinoCommon] RTree is too limited
Another thing we have in c++ is stable multi threaded point projection to a Brep. Coded in c# with TPL this would crash randomly.
I was hoping to test our c++ code at some point with the WIP.