It's been over 20 years but my grandson and I provided tech support for a big cross state ride.

As you might imagine, flat tires were far and away the #1 mechanical issue.
Broken spokes was, by a large margin, #2.
#3 surprised me - it was bottom bracket repairs and servicing. I did a couple of roadside hub overhauls too.

I replaced both shift cables for one fellow that were so seriously frayed they looked hairy where they exited the shifters. I asked him: "When you signed the form where it said your bike has been recently serviced, what did you think that meant?