Seen that before, good positioning. I only foresee three challenges:
1. Metal there isn't too solid so you may have to add some sound deadening to get the best out of them.
2. Decent 6x9s and MDF are heavy and that weight is at the outer most part of your tailgate so you may find it won't stay up depending on how good your struts are.
3. Routing cables to them- it may be a little on the fiddly side and you will need to wrap them in spongy tape to avoid buzz.
But as I say, plenty of people have done it, so it's very doable!
I'm opting for decent side speakers combined with my small concealed sub, so will let you know how that sounds when they arrive and I fit them.