d4!?

Working in the mentor and this came along as I was looking at defenses against the fried liver.

They talk about how Nxe5 is not a good plan:

" This is where many beginners go wrong. It seems only natural to capture the pawn, but that leads to big trouble after 6.d4! or even the famous Fried Liver Attack 6.Nxf7!"

What's the big trouble of d4?  Is it that white's bishop is now engaged?

Please correct me if I'm wrong but 6. d4 blunders the pawn because of exd4.

if exd4. 7. 0-0, Be6 8. Re1, Qd7 9. Nxf7, Kxf7 10. Qf3+, Kg8 11. Rxe6 and the rook cannot be taken because of Bxd5 winning the queen. white is easily better and probably winning in every variation

Smart! Guess you learn something new everyday. :)