Weight restrictions stink, and it's hard to know. I was once waiting 3 days in ATL to go to NRT, and didn't get on. On the 3rd day, we nearly made it, but then dispatch sent over a tire (a tire?) to be hauled over. Freight takes precedence over non-revvers, since freight earns revenue. So, on that type of flight, you will see lots of availability on non-rev, but that does not mean you have a good chance of getting on.