You should be aware of the complications that can occur when putting a boat on a roof rack, including the handling differences that your vehicle will have (such as a limited ability to safely swerve, or brake in an emergency) but what you've been told is beyond concern, and goes into the realm of fear. You shouldn't be scared to roof rack your boat.
Here's what you need to know.
A modern day Yakima rack (with Q towers) will not only grip your car where the BLACK arrows show, but will also hug your car as shown by the RED arrows. Modern racks are MUCH more secure than older racks if they are installed correctly.
Secondly, attach a bow and stern line. These are designed to do two things. First of all, if a boat, or the rack does somehow manage to break loose, the boat will remain on the roof of your car, and give you time to pull over. The second thing they are designed to do, is prevent that lifting force in the first place. The bow and stern line should be snug, but not tight. You don't want to bend your boat, but you DO want to limit the amount of lift that can occur.
I always prefer to use cradles. I use the Yakima Mako saddles. They act like a suspension to the boat, allowing the boat to move just a little bit over rough surfaces, but also, when the boat does feel like lifting, the slight give in the saddles actually prevents the forces from putting bending forces on the boat, and instead will slightly compress and decompress the saddles.
To get an idea of what the Mako Saddles do, picture them as 4 of the Beach wheels from a Hobie cart holding the boat. If you lift on the front of the boat, the rear wheels will slightly compress, which transfers the forces from the boat to the wheels, or in my case, the saddles.
Finally, I've taken a complete AI, with Ama's and sail on the roof of our old Civic, and still had a Revolution beside it. No problems with the lifting forces, the rack never budged. Do yourself a favour and get a modern rack, install it correctly (which is simple, but important) and keep your speed reasonable, and you'll be fine.