Most but not all USB pendrives are reliable for booting, even many of the slower ones, and they are much cheaper, and should be OK particularly for regular read-only live drives (without persistence).
Some computer hardware and some operating systems have issues with certain ports. And some USB pendrives just have issues also. Some of them cannot be used for booting. They are made to be mass storage devices, and have not exactly the same electronics and firmware. Some USB pendrives and computers 'do not like each other'. The pendrive might boot another computer, and the computer might boot from another pendrive (everything else being the same).
This is a link to test by Pendrivelinux including bootablility of USB flash drives. This test was made a few years ago. The cheap and slow
Sandisk Cruzer Blade, 4GB, can be added to the list of reliable pendrives for booting. I have used it extensively for years and it has failed only once (chainloading from Plop in a very old computer).
This link shows a bootability test in January 2014.
Some pendrives that did not work are shown in
this link. This user is not the only one who likes
32GB Sandisk.
