it's not a bad design, but instead we had the problem of first adopters. not just early adopters -- innovators. in a world of monarchs and more monarchs, america was unprecedented in almost all ways.
over the years in order to deal with changing times we've relied on the courts instead of the amendment process (aside from a few small things when the court was horribly wrong, eg, dred scott) but i think we've reached crisis stage where we need to really have a good think about what we want out of it
in terms of federalism, i guess the main points that ought be raised is what level of independence do we want states to have in a world where services by the state are thought of as obligations by the state in most parts of the western world. are the states laboratories, or are they unruly children who need to be put in their place? america is so unique and diverse i think devolution of power from the central government makes a lot of sense and adds to america's dynamism. canada has a population less than one US state, and as big as the country is, everyone basically lives next to america. and even still it has problems with separatists because of the french influence. america has a massively diverse population with lots of unique strengths and problems to solve. i think states are better equipped for this
that doesn't mean that we shouldn't have a rethink on the consitiution. i believe we should. but we need to examine the positives and negatives to centralized authority