Generally, you only find out how bad after the accident.
Directional tires handle substantially differently when mounted " backwards".
If you're going to have wonky handling you want that on the front of the car where you can control what's going on. You do not want the rear of the car to go off on its own path.
Don't mix tires even of the same brand. Tire manufacturers don't even like you replacing just one tire because the safe handling characteristics of the tires are generally tested in sets of four fairly evenly worn tires.
I strongly recommend against mixing tires on the same car, let alone the same axle. If you must mix tires be aware that the different tires may behave quite differently under different conditions especially on wet or snowy roads. You want the lower grip tires on the front so the car will understeer in an emergency.