J.R. is right on target. The benefits, obviously, are that you have a wider selection of properties, including some (some FSBOs) who aren't identifiable on the MLS. And that's really about the only advantage. If you've been searching for a while and haven't had much success in finding what you're looking for, then, sure, definitely expand your home search.
The drawbacks are many. FSBOs are often overpriced. Partly, that's because some are already financially squeezed and need every penny they can get out of their property. Partly, that's because some are still into the 2005-2006 property price mindset. And, without trying to psychoanalyze FSBOs, a lot of them are very self-confident. After all, they're convinced they can sell their homes themselves. (And, yes, some can.) And sometimes this self-confidence runs over into the pricing process. Honestly, I've seen a lot of very nice looking FSBO homes. They've been modernized, they're fixed up, they show well. And the owners know it. Unfortunately, they sometimes overestimate the value of their work, and so overprice their homes. There's also the question of, if you're using an agent, how that agent will get paid. Sometimes, FSBOs will pay the agent's fee. Sometimes not.
As for the drawbacks of foreclosures, first there's no assurance that foreclosures are good values. Second, although not as bad as short sales, foreclosures are unpredictable. It's not like making an offer on a regular home, or even a FSBO.
Hope that helps.