When talking about inspections, I agree that the border agents won't do anything themselves. I will say that in New Brunswick they may hold the vehicle and call the Commercial Vehicle Enforcement folks. They have mobile mechanics who can do a safety inspection of a vehicle right there. They may be named "Commercial" but their mandate is to enforce the motor vehicle act. In both New Brunswick and Nova Scotia they will have these folks at road side police checks and do safety inspections right there. A vehicle that is unsafe can be towed away.
I haven't heard of it happening personally, just know it would be possible for them to do that.