I have built several beds and they ARE about the hardest. Not because of the build, but because of the loads comming from several ways. One is movement from the sides, back and forth, kids jumping on them, etc. Then there is the loading...is the box springs going to go inside the box or be on top? One has the side boards wanting to twist from from the load the other has downward load. If you want looks would a finger joint be ok? Because you could double mortise the ends of both boards, then finger joint them so they come together. Glue the tenon in one piece after that, and use a draw bolt to pull everything together, so you have looks and strength.