Plenty of reasons for that: -To much particles - when you slow down the game with the ingame slow-mo, you will find much more particles, smoke, etc than there would be in a normal speed crash. No idea why. -I have to press 10mio buttons to get my camera movements and mutlivehicle control working, cheat engine uses 1 click to get the exact speed I want, for the ingame thing you have to use both hands wich I can't because I one hand I have the controller. Ex: If I use cheat engine I have a simple layout: pressing "1"equals game at normal speed, "2" = 0,5 time, "3" 0,1 time, etc... If I would use ingame slow-mo, it would take me a few clicks to try and get my perfect speed. -Camera movements stay at the same speed. If I use ingame slow-mo, my camera movements will be way way to fast as it slows down only the vehicle. Cheat engine slows down the whole game so the camera will be exactly that much slower as the vehicle. (I know you can set the camera speed in the editor, but "5" is still waaaayyy to fast for a crashing vehicle at only 0.1 time of the speed.) There are still a few small other points but those are the main ones. @Edit: Also, no idea why, ingame slow-mo doesn't give you much fps improvements, at least not for me. So sometimes I had 5vehicles crashing in slowmo and only 20-30fps. Cheat engine gives you absolute beast FPS, like slow down your game by 0.1 and you will get a few houndred fps even with 10vehicles crashing.
1. Google 2. Of course, the game will be running at 10% it's original speed, it's unlikely you won't notice.
Oh what have I done It takes a while to master, but don't worry, I have crashed many, many, many, many, many, many, many, many, many, many, many, many, many, times.