I am curious why I keep reading that these mowers are not good for hilly lots.

I have used zero turns extensively but a long time ago.

I don't remember them being particularly dangerous if you use common sense.

Is the problem with going up and down a slope or trying to mow horizontally along a slope?

I run a small mowing business and am going to buy a zero turn mower to increase my production.

I am looking at a Toro or John Deere 42 inch cut, any advice?