>How lower, like by factor?
Canola oil is approximately one-third polyunsaturated fats, containing a lot of linoleic acid, which is broken down to arachidonic acid by the body. Arachidonic acid isn't bad in and of itself, it lives in cell walls and plays a key role in how the body reacts to damage or how an immune response is triggered, but excess amounts of it essentially keep your body in damage control mode - it turns the excess arachidonic acid into inflammation-signaling molecules like prostaglandins. Meanwhile beef tallow contains on average less than 5% polyunsaturated fats.
less prone as in...?
Saturated fats and monounsaturated fats have no/fewer double bonds. Double bonds are vulnerable to oxidising because they're electron dense, and oxygen when heated forms reactive species that then are drawn to/'attack' the electron-heavy double bond. Which causes the bond to break, which causes the release of free radicals, aldehydes etc. This of course ties back to the same factor of polyunsaturated fats to monounsaturated/saturated fats as discussed.
no solvents, how can you be sure what is added
Because there's no need for it. There's no role for it to play. To make tallow you render fat and run it through a sieve, there's nothing a bleach or a solvent can do in this process that contributes in any way.
That's the end of the lesson. If I have to tell you why it's bad to have your system flooded with low levels of prostaglandins at all times, or why it's bad to have a lot of aldehydes in your food, then I'm simply not going to. It would be damaging to me to force myself to try to think at a sufficiently primitive level to communicate these concepts to you.