I never contribute to these discussions, but I think there should be some clarity. The expression is not about problems at all. It does not describe when the work begins, either, but rather where the work happens. There is a big difference there because the expression does not denote anything about the timing of the work. It does isolate where, exactly, effective work happens.
For example, In higher education, there are administrators, regulators, support staff, but in the classroom is where the rubber meats the road. I guess the other parts are thought to be in the engine. The supperintendent is driver, perhaps; nowhere near the actual place where education really happens.
You can replace it with, "where the work really happens" or "where actual progress is made". It also connotes the more menial work that goes into a sizable undertaking.
For example, There may be designers, manufactures and managers, but in the call center is where the rubber meets the road. In this example, manufacturing is definitely work, (perhaps the "rubber meeting the road" for design work), but the call center is where the interaction with consumers happens. Though not as glamorous as any of the other positions, it is where the rubber meets the road.
In military terms, the rubber meeting the road happens when boots are on the ground. :)
Hey, hope that helps - H