Just to be pedantic, it's not a "so called limit" it's a limit.
I've heard that it can give you an upset stomach/headache/diarrhea. None of that sounded very pleasant, so I opted to stay at the 156 ppm level for comminuted meat. I prefer to use the scale for small recipes as a small amount over can make a rather large PPM difference. So if you use 1/4 tsp per lb, you're good at 206 ppm, although like I said before, a teeeny bit high. However, say it's maybe a bit more heaping than normal so instead of 1.5g, you're using 2g. Now you're at 275 ppm, which is starting to get up there.
The other interesting thing is that the USDA doesn't give you a minimum, just a max. I gather because cold storage/freezing works fine to preserve meat with no additives. I've vac sealed whole muscles with cure (pork loin for instance) at the 156 ppm rate and left them in the fridge for I think 2 months. Opened the bags to some very fresh smelling meat, probably smelled better than when I put it in there. Obviously that's how it's supposed to work, but I was really impressed at how
well it worked.