USB-OTG vs portable Sdcard
First thing is to verify if the device you propose to buy has USB OTG support. Apart from verifying from OEM specs, do see this How can I determine if my device has USB Host Mode (OTG) support? and also the listing  of supported devices by Chainfire. Saw your edit about Nexus 7. This post confirms OTG support on Marshmallow
For your stated purpose of storing ROMs and backups it should work well. If you are using TWRP recovery , this thread may help, TWRP Using USB OTG to backup? 


I have seen my friends  use it thus but haven't used it personally- all my devices had external SD to store