You're absolutely right. I think the biggest barrier would be brushing up on these topics. I haven't touched these subjects in 5+ years. Teaching requires one more plane of understanding.
Dave, UPchieve CTO here. A really cool thing that's happened over the last year is we've gotten lots of volunteer sign ups from retired people, who were really trapped at home from fear of COVID, who have lots of time to brush up on physics/calculus/etc. I don't think that's something we could have predicted about the marketplace of students/volunteers but has helped a lot. Even now that things are opening up, they're staying with us and continuing to pick up lots of sessions. They enjoy the mentorship aspect and brushing up on the academics is a fun thing for them to do.
We give volunteers review materials to help them brush up, and to be honest... even the best tutors have to Google things occasionally to refresh their memories during a live tutoring session. :) But yeah, it does take some investment on the part of the volunteer if they haven't looked at a specific subject in 5+ years.
I think that’s a feature, not a bug - part of the reason I’d like to tutor high school calculus students is so I have an excuse to revise that stuff again - I finished an MS in math and statistics last year, but it’s probably been 8 years since I last did high school / freshman calculus :)