I hadn't thought of the "residency arbitrage" problem at the state level. Presumably it would work a lot better at the federal level as a result; we already have lots of effort put into both becoming a citizen and determining who is and is not a citizen.
The other benefit of a negative tax (negative consumption tax or negative income tax) is that we could then get rid of virtually all other benefits and entitlements -- just increase the base subsidy. There's no reason to administer a food stamp program if you just give everyone a $20-30k/yr income. Same thing with student aid; why give someone subsidized loans when you can just give them $20-30k/yr and tuition/etc. costs $10-40k/yr, so a feasible part-time job (or private scholarship) covers the cost of going to school at Harvard, and you can actually save money if you instead go somewhere cheaper.
I'm not 100% in favor of giving everyone free stuff, but I'd far prefer to just give everyone free stuff vs. build an expensive bureaucracy to administer a program which incentivizes poverty and disability (a huge fraction of the SDI payments are to people who are basically employable and functional, but who can't get jobs; if they had an independent $20-30k income, there would be nothing to discourage them from doing $5-10k extra in legal work.) Obviously services other than pure transfer payments would still be needed, but the majority of welfare programs are about the transfer payments; counseling and other support could be charity, commercial, or funded separately.