None.
Requiring nothing but a "coalition of the willing" doesn't constrain us at all.
Exactly.
We have never needed "UN approval." Only leftists traitors and foreign enemies want the USA to be constrained.
Just realize that when we act unilateraly we set the precedent for other countries acting unilaterally. This precedent may come back to bite us when the day comes that we are no longer the strongest nation in the world. Which since history tends to repeat itself will likely come to pass in the next 2 to 3 hundred years if not sooner.