http://cvp.com/index.php?t=category//tripods+||26+support/vehicle+mounts
Obviously three point mounts are more stable, though single points for small gh4 style cameras work. Gaff tape the lens focus, run an hdmi cord to the inside of the car to run a check as you work.
Up to 40 mph on bumpy London and LA streets your fine, above 50mph three point is safer. If you plan on going faster, then there are ways to make a safety strap, using straps that are gaff taped down round, not flat so not to catch airflow and vibrate.
As you know clean the surface of the vehicle and suction mount and then with a damp sponge slightly wet the surface for better suction, though every 40 minutes you need to check the mounts for dryness and vibration.
I've run one up to 75mph, 50 mph inside LA, 40 mph inside London without issue.
Bottom line, more you spend, safer it gets, though it will be obvious.
I haven't use the chinese camtree system
http://www.proaimshop.com/pas/Camtree-Gripper-G-91-with-Suction-Cup-Mount-Dust-Cover-and-Elastic-Wire.htmlbut do use their shoulder mounts for our reds and love them.
IMO
BC