The US can show the problems of a weak federal system without the current US government being a weak federal system: the system under the Articles of Confederation was a very weak federal system; the system under the Constitution prior to the Civil War was a weak federal system. Both the issues that the former produced that resulted in the Constitutional Convention and the latter system's end in the Civil War can be viewed as illustrating problems of a weak federal system.
I understand why one might (or disagree that the failure of the AoC is a good illustration of the same point) -- my point was simply to illustrate that the "no need to look farther than the US" proposition does not require the current US government to be a weak federal system. Whether particularl issues that occurred under the past weak federal systems in the USA (including that of the CSA) each really illustrate problems inherent to weak federal systems or whether each is simply an example of problems that happened to a country with a weak federal system that aren't inherently related to having such a system is a whole series of other arguments.
I'm not sure if this would have prevented the Civil War. The Gilded Age robber barons would eventually have amassed enough economic power to challenge the southern planters. If they avoided the slavery question, the Union might have held together until mechanized farming made slavery obsolete, perhaps during the 1940's. Northern industrialization would likely have involved more turmoil. The U.S. might also have taken over various Caribbean and Central American nations. I doubt this situation is an improvement over the real historical one.