Is it a good idea to run MacOS off of a flash drive?
Yes and no, Let me elaborate. Most USB 3.0 drives could run it almost as good as an hdd, If you have a SSD, that would be the best option and is usually faster than the hard drive the comes with any mac. Usb 2.0 can technically work but is very slow as the read/write speeds aren't the best. Im currently running mac os x 10.11.6 on a usb 3.0 flash drive i got from walmart for $17. I will mention however, that the newer OS your using, The slower it will be on os x. I still think that USB booting is great though because of the portability. Just install the OS on the usb flash drive, and hold down alt at startup and it should work perfectly fine. If you have any questions or want me to make you a bootable image for your flash drive, hmu.