Hi Eric,
There are no big penalties, the only real problem is that edge schema is not validated because lightweight edges are not instantiated as ODocuments, so the validation does not run (eg. if you define a mandatory property on the edge class, it won't be validated with lightweight). For the class hierarchy, it's exactly the same as regular edges
We mainly suggest to use regular edges because it's easier to deal with real records, with RIDs and all the rest, rather than having to figure out if an edge is regular or lightweight, especially if you work with document API
Thanks
Luigi