Breaking is certainly an issue.
Equally important would be things like how smoothly it works, how powerful the motor is, and, as Kooter mentioned above, how wide the belt is.
Many years ago, I made the mistake of buying a $100 stationary bicycle. It was unbearable to use. I finally just hauled the almost new bicycle to the dump – I didn’t feel I could in good conscience sell it to someone else.
I absolutely hate exercising anyway. For me, having a poor piece of equipment becomes a real good excuse for not doing it to begin with.
A couple of years ago I bought a $2000 True treadmill (it was so cheap since it was a discontinued model!). It is a wonderfully smooth and powerful piece of equipment with all sorts of bells and whistles! I only use it for walking, and, admittedly, it’s definitely more than I really need.
However, despite the very substantial cost, I’ve never regretted the purchase and would do it again.
I’ve long since lost the links, but before getting the above I read a lot of reviews of treadmills on the internet. I don’t recall anything under $600 or so getting an even marginally decent review.