Flakes are pretty easily managed, but they can also conceal a bigger problem.
Generally where it matters the most is near the power steering box. you can get cracks in there, and the cracks can make the rust even worse.
Keep an eye out for big bulges in the frame where the rust is coming of in layers hiding a bigger problem underneath.
Most frame issues can be resolved with the right tools/expertise. Take a good long look, and if you see something you're not comfortable with... negotiate a better price and buy yourself a welder! ... or something.
