Recommended slopes for boat ramps is between 12-15%. At the high end, it would be about 8-1/2 degrees slope. For a 5000 lb [2273 kg] boat/trailer combo and 15% slope, and with zero rolling resistance, it would take about 740 lb [336 kg] pull to hold or roll it at constant speed (holding is a constant speed of zero
Add in 10% rolling resistance (could be higher), or 495 lb [225 kg] and you get 1235 lb [561 kg]. Theoretical. Actual rolling resistance may be higher, and I'm guessing at boat/trailer weight. And you still have to accelerate the load from a standing stop, which adds load to the strap.
Many tow straps I saw online have questionable ratings. They seem to suggest the weight of the vehicle you can tow on flat ground (my interpretation). Others have a minimum breaking strength, which is more meaningful, like this one https://www.amazon.com/gp/product/B01H5 ... i=15737581
but it's over $50 US. The $11 US straps with the 10,000 lb [4546 kg] ratings sound fishy to me. https://www.amazon.com/gp/product/B011A ... i=15737581
and I'm guessing that's some sort of meaningless towed vehicle weight figure or something.
And as kadet correctly points out, they're big rubber bands, and may do a lot of damage if broken from overload. Kevlar is stiffer than steel rope, but crazy expensive, and wire rope is still stretchy, so that danger is there no matter what you use. Even chain is stretchy.