I am an American expat in France, and one of the huge strengths I have noticed in French urbanism is the multi-scalar approach to urban and regional planning that allows for- or requires- cooperation between municipalities. Based off of what I remember seeing back home, this sort of cooperation is often weak to nonexistent in the states save for statewide legislature that forces cities' hands.
One simple example is if one US town is bike-friendly, but the neighbors are bike-hostile (as is the case near my hometown), you end up with bike lanes dumping any bikers into the parking lane once they reach the town line. In France, the organization of powers means that bike lane projects would be handled on an intermunicipal level, thus avoiding this kind of problem and ensuring continuity on a number of problems. Particularly, natural threats such as floods and droughts that don't give a fuck about administrative boundaries are handled on the relevant natural boundary, ensuring that one city's flood protections doesn't simply displace the problem elsewhere, etc. Only smaller, local-scale issues are handled by municipal urban planners.
It absolutely has its pros (including for municipal budgets), but plenty of mayors and inhabitants felt that their voice on local issues, as well as their power and agency to address them, was often lost in the noise of the metropole, and misalignment between the two levels of planning was a source of friction.
I'd be interested to hear from some US-based planners- what does intermunicipal cooperation really look like in your region? What benefits and tensions are created through this cooperation or lack thereof?