If you live anywhere with winter, road salt, or even extended periods of rain, you will become sympathetic to just how unforgiving rust can be on nuts and bolts.
Undoubtedly the number one method for loosening up rusted nuts and bolts is to use heat, normally in the form of an oxy-acetylene torch.
Oxy-acetylene, has a few hurdles:
1. It uses a flame so it cant be used around fuel systems
2. Generally expensive to initially acquire
3. The fuel tanks make it cumbersome
4. You have to maintain and refill the oxygen and acetylene tanks
An alternative and in my opinion better tool to use is an induction heater:
2. Directed only at Metal
3. Can be used cautiously around fuel systems
4. Plug and go without any maintenance
5. Reasonably affordable