Here for me: small USB flash drives and larger USB hard disk drives (640 GB portable, 3 TB desktop) seem fine.
Very recently I began using a very small USB flash drive as a cache vdev for the 640 GB portable. Under
ZEVO resilience to untimely loss of L2ARC we have an example of ZEVO apparently working well enough after I removed that flash drive
without an unmount of the file system of the portable. (Beyond the proof that it works, I probably gain no performance from this class of flash drive as a cache. The thing is so small that I often can't find it or treat it carelessly – like, snugly zipping up the Brenthaven case without realising that I forgot to remove the drive from the MacBook Pro. Note to self: slip the flash drive inside the rubber case of the hard drive.)
As you're a newcomer, welcome :-) and a hint: if you have not already done so, please apply the update to 1.1.1.