Some cars have a maximum permissible towload even below the 85% guideline. Manufacturers also sometimes specify maximum gross train weights that are less than the combined sum of the gross vehicle weight and the towload. This complicates matters even further.
It all depends on what the tests that the manufacturers carry out to establish the towload come up with. Standards specify that the car when fully laden must be able to start five times within five minutes on a 12% gradient. The limiting factor here is often engine cooling, especially as more and more components are crammed under the bonnet of modern vehicles. Clutches used to be a problem in the past but, with improvements to design and materials, one rarely hears of this nowadays. Endurance tests are also covered in the standards (2 million durability cycles are required) and these could put further limitations on towload based on what the car's structure is able to take. In addition to what the standards require, the manufacturers also have their own specifications and these could cover handling and braking performance, for example.