It seems that at least those sticks I have, are not that bad.
I tested the first stick which failed first on Solaris 10u9 and the second stick with another FreeBSD machine, and both do not show traces of problems there. The first stick attached back to the machine which exhibited the problems initially shows problems again.
To determine if it is the USB hardware or the FreeBSD USB kernel subsystem, I will step by step update the other FreeBSD machine which did not exhibit the problems to more recent versions of FreeBSD (binary search) until I encounter a problem with the USB stick (or arrive at the same FreeBSD version as the machine with the problems).