Adding to jggimi's informative post, one problem you might encounter is geometry related.
When a system boots from USB, the BIOS will emulate a hard drive temporarily for real mode disk access.. this allows the MBR and the PBR code to read and write from/to the USB mass storage device.
Sometimes this emulated device will have geometry that's different from what was determined via the umass/sd(4) probing, the side effect is the second stage loader cannot find /boot on some systems. (..you'll likely see "ERR M" if this is the case).
I've only witnessed this on some older systems, but you might scratch your head wondering why the device boots on some systems.. but not others.
Hope that helps.
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