I see lots of people around the bay area with that tailgate thing. Seems like a great way to scratch up the paint of your tailgate. Car covers scratch your paint if you leave it on for months. dirt gets under there then the wind blows. That tail gate thing has to be 10x worse than car cover with the weight of the bikes shifting around while driving over bumps, turns, stops etc...
No personal expedience though. Maybe someone can chime in that left it on for a year and took it off to see how the tail gate held up.
I would use a trailer hitch kuat bike rack or one up bike rack if the truck was shinny and in good condition.