According to Aerosoft’s Mathijs the MSFS SDK is now complete to the extent that you can do pretty much anything you want to do with it:
"Getting C++ code to run in the sim is not really a problem. You can make a electrical or pneumatic system system as complex or deep as possible. Beyond what any P3D add-ons has now if you so require. It’s just code. Of course there will be problems as variables from the new sim do not give the same value as on P3D, but you ask Asobo for help and you will get is as long as you are in the correct cycle. You might need to wait for the next update of the sim to get it running on the sim customers have though. But as I said, when needed Asobo/MS will update when needed.
I will give you an example. We have a professional project intended to train people. It is about the electrical system of an aircraft that shall not be named. Very serious stuff. I mean it models the reduction of load when a footwell heater fails. Pure C++ code and and while the output to the displays needs to be redone, it all works without any issue in MFS. 100% the same code as in P3D, just needs new variables in input and output. it can’t be used in MFS as you can’t get professional licenses but if the customer wants proof the code runs we are happy to show it.
As I keep saying, what you hear about more complex systems in the new sim has as much to do with commercial interests as it has to do with what is actually possible"
P.S. Thanks to @CptLucky8 for PMing me with the correct link for the source