'Oftentimes' isn't something I hear or use in everyday speech. It is a little formal and I think I mostly hear it instructional environments. Another word that can be used is "usually." If I was formally discussing something, or in an instructor role, I'd use the sentence structure of the first two: "Quite often, people underestimate..." or "Oftentimes, people will underestimate...". But if I'm casually talking I'd be more likely to say, "People often/usually underestimate...". *Not a teacher/professor, just providing my experience. Hope this helps.