i bought a low profile sandisk with the intention of loading it with music and leaving it in my truck. left it plugged in for a number of weeks before i noticed it gets HOT, it wasn't being used, just left plugged in to a port.
in my opinion the form factor is not worth the trade offs, i haven't used that drive since.
I agree that the heat is of concern and I cannot say that I am personally very comfortable using something that gets so insanely hot for prolonged periods - especially in regards to allowing it to operate when I am not present (which, ironically, is the very usage for which many of the ultra-compact flash drives are marketed for.)
I've noticed this too with that particular UltraFit model - even during idle periods, it gets HOT. During prolonged active usage, it can easily exceed 130F (and some Users have measured surface temperatures at high as 175F - amusingly beyond SanDisk' own stated limits of safe
non-usage storage temperature, and way, way,
WAY beyond the stated limit of maximum safe operating temperature of 135F!!!!) As this is enough to cause thermal burns,
it is clearly something of concern to those with younger children or pets within proximity of the device, and IMO anyone purchasing this product should be aware of this beforehand. How this affects the longevity of NAND, or how this could affect the device it is connected to and raising the temperature of, I can't say, but I have noticed that there are quite a few reports of premature failure (this could easily have nothing to do with the heat and everything to do with lower quality materials used in most entry level consumer flash drives today.) But I can't say I would be comfortable leaving it in a device for a prolonged period, or using it at all when unattended.
SanDisk previously had a USB 2.0 version of the low-profile UltraFit called the Cruzer Fit, and I do not recall it getting nearly as hot. It was a reliable drive, although it had comically slow write speeds (not as an outcome of using USB 2.0, but as an outcome of the NAND itself.)
I find it very amusing how, since moving from USB 2.0 to USB 3.0, many flash drives have gotten slower in regards to writes. But Makers realize that emphasizing USB 3.0 is a great marketing strategy in that consumers automatically assume the flash drive is much faster, which it may or may not be depending on usage. Now with the shift from USB-A to USB-C occurring, Makers will probably market USB-C flash drives as if they were somehow faster than USB-A flash drives (which they are not), and will use the new USB 3.1 branding to imply the drives are faster than USB 3.0 (even though 3.1 gen 1 is simply the new terminology for the exact same protocol formally known as 3.0.) And sadly, I fear the few remaining higher quality flash drives on the market will likely go the way of the dodo, given most buyers don't really think about the quality or longevity of the product until the drive actually fails. :sigh: