if the USB stick get infected by a virus, the virus might delete (or corrupte) wallet.dat file
so I recommend burning in into a CD using a burn studio to deny viruses from doing anything to the file
But the USB stick would have to get infected from something. So if the USB get's infected your computer is infected and the USB stick was infected second.
So no matter what usb stick or cd. The infected computer could compromise the wallet.dat (or other files). When they are being loaded on computer. This is why you really should only use it with a clean computer. If you use one that you surf or worse torrent with there is a bigger chance of it being infected.
If a USB stick is unpluged and cold storage it cannot be hacked unless host computer is compromised when it plugs in.
If you wanna hold the wallet for long term (and its a deterministic waller), then you burn it into a CD to avoid cryptolocker and other shit.
If you use the wallet frequently then an USB is good, but always have multiple backups in case of a cryptolocker infection.