I have the same situation as the OP (although my two USB sticks are brand new). As a newbie I would be interested how you know this is the problem - is it just the voice of [bitter] experience?
Also, is it possible to determine the exact nature of the failure? In my case I'm using an old m/b in a home assembled machine (I used to be in the business so I have a good idea of what I'm doing) and it could be a dodgy m/b eg. the USB ports on the m/b might not be up to scratch. I don't want to keep buying USB sticks if the problem lies elsewhere. In saying that, the USB sticks might be a good place to start - they are cheapo Lexars.
When you use ZFS on the boot device(s) (which, you will be, on FreeNAS 9.3 and later), it will, of course, inform you when it encounters read, write, or checksum errors during regular use, or during a scrub. There's nothing else, really, that can be the cause. Either the flash blocks read, write, and checksum correctly, or they do not. The beauty of ZFS is that it will tell you when this occurs. With the Kingston Micro DT, which used to be my favorite device for this, one we went to ZFS for the boot pool, then 100% of my deployed Kingston Micro DT's showed up with errors in a short period of time. Same for a variety of other devices I tried (though, not 100% of them). The only one that I have used in my systems, and deployed in friends' systems, that I have not had a problem with is the Sandisk Cruzer Fit 2.0.
The nature of the failure? The nature of the failure is that the blocks did not write, read, or checksum properly. i.e., poor flash or firmware (whatever constitutes firmware in this case).
I think the reason it's not a big deal for other people, is that they're doing things like storing photographs and stuff on these devices, and a bit or two out of place here and there is probably not fatal; probably in most cases, the read/write is retried, possibly succeeds, and no one knows it was jacked up with something like FAT. And also, of course, when you use the device as a boot device and have it plugged in 24/7, it experiences a bit more usage than Aunt Sally's SD card in her camera, giving more opportunities for errors to creep up.
Anyway, yeah. USB thumb drives are not, typically, precision/reliable devices, and none of us expected how blatantly this would be revealed when we went to ZFS on the boot pool.