As I said, you will run into access violations when you instantiate your fields on the class instead of the Init function. And as I also said, in your concrete case it would probably even be fine if you would directly do it in the class scope. But I would advise against breaking this rule. There is nothing to optimize here, either return a new instance when this is only called once in a blue moon and you do not want to bother with initializing a field. Or define a field and initialize it up in NodeData::Init. And no, scene elements (NodeData) should not share data on their class. We have types in the Maxon API to deal with this when you really want some heavy data structure which shall be initialized exactly once, but that is total overkill in this case.
Cheers,
Ferdinand
