Another vote for the Altera Strada
Positives
it has a system that slides it away with a tilt, been using it for years and one of the best bits of kit I ever purchased. Takes seconds to mount on the ball and you have to lock it on plus it has locks on the bike strap arms.
It folds up so easy to store.
I would buy another one….
Negatives
The space allowed between bikes when you have 3 or 4 loaded means some protection may be required, but have experienced this with other makes so a common problem.
The arms that hold the bikes have to go through the frame, this can be interesting with some frame designs and piggy back shocks…. Also a problem on othe racks with the same system!