There are lot of loopholes, if you go in detail, so much so the RR Martin is working on preQual to GOT, events leading upto GOT, we may see that as well in future, now since GOT Brand is so well known...We may not see everything explained even in season 8. We already see lot of back reference in Season 7 to past kings and events...
To me big one is, why WWs are back in action after 4000 years, neither books nor show explained their return. They have been dormant for 4000 years, something triggered them to be back in action, what was it?
They all heard about those supernatural creatures but seeing them in action is whole different stories. Imagine in our world, religious supernatural come into action, it would be something like that. Its hard to believe unless you experience yourself...
Incest in Targaryen family is normal, going out of family is unusual