Is it safe to transport a rotomolded kayak on the roof for several hours in the hot sun?
Sure. Just use cam straps, not ratchet straps.
I preferred to carry mine upside down when I used to carry it sitting directly on the rack bars with cam straps and bow and stern lines. Figured if it was going to get any contact areas slightly flattened in the heat I would rather it be on the deck where it would not effect performance. Also did not have to worry about it filling with rain if I ran into a storm during the drive.
Now that I have Thule cradles, I feel like it’s safe to carry it upright, unless rain is a possibility.
Rotomolded plastic gets soft in hot weather and/or in the sun. All is fine so long as you don’t make dents in the kayak.
Ratchet straps are known for over-tightening, which is why they are recommended against. Cam straps are less likely to be over-tightened.
Putting the boat hull side down straight on a bar without padding is also bad. Saddles or J-cradles or putting boat in its side with “stackers” (which is much stronger plastic than the bottom) are ways to avoid this.