Yikes indeed!  Some factory roof racks are cosmetic.  Some are good for lashing your skis to, but not much more.

In addition to ensuring the canoe is connected to the actual frame of the car there is also the concern of opposing forces.  If your bow/stern ropes are running more parallel than intersecting then those forces are pushing the canoe towards to hood or the trunk.

Having girdle straps that are fore & aft of the widest point of the canoe and tight to the contours of the canoe will also keep it from sliding.

I'm never happy about a canoe that's not tied to the frame of the car.  Once I've car-topped my canoe I grab and shake, right & left, forward & back.  Do this from the mid-point & ends.  If I can pull the canoe off the car it's not safe.  If the car's a rockin' I'm ready for the road.

Open fields & lakes can whip up gusts.  Watch the trees & grass & crops & waves for winds.

Occasionally stick a hand out the window & measure how far the canoe has shifted.