I like the idea of removing power from the federal government in favor of states, but in the case of a service like social security, mobility between states could cause massive issues. State x would not have a safety net, prompting companies and workers to operate there for the tax break. State y, providing a safety net, would be swamped with retirees but not productive workers. Citizens would be hugely incented to game the system. I don't see how it could be sustainable.

If the discrepancies between state policies were large enough, I imagine you'd basically see white flight on a national scale. Essentially, supporting any type of social safety net would be optional for the people with the resources to move out of state. As a result, middle-class and wealthy people would flock to low-tax states without expensive social programs, leaving behind a higher concentration of needy people in the states with the stronger safety nets. What social programs remain would presumably go bankrupt while the nation's wealthiest individuals basked in the glory of their virtually tax-free lifestyles.

That is largely not the case with your neighbors to the north. In Canada only 1/3 of taxes are collected at the federal level, and some of that money is distributed to the provinces to spend (poorer provinces get a larger proportion of payments so they can provide similar services as richer provinces).

While there is some migration to the richest province, that has more to do with jobs related to the oil boom there, and the low taxes there are due to oil revenues.

One possible counterargument to the state-specific approach is that if you have a handful of states that really do well, a majority that do okay, and a few that really screw things up, it's unfair to the residents of the states that chose poorly. They may never be able to move to a better state because the decisions of their current state left them in a financial position too weak to afford life in a different state.