Hi @Stogie, et. al.
There has always been a few SIGNIFICANT differences with the hardware being discussed in this thread. They are System Hardware, Life, and Performance.
Firstly, USB ports and its serial nature do not compare with PCI ports. This has been known thruout this century. The ports are design differently. The performance is vastly different in these 2 types of ports with PCI (for internal motherboad connections) being superior by far.
USB memory sticks and SDcards are built with low cost materials to make them attractive for storage use. System drives designed for direct motherboard connections have a very different design criteria and lifespan. As such, their life will, in almost all current cases, exceed the life of sticks and cards.
For full OSes, the OSes are designed to take advantage of the presence of storage, so for full, non-RAM OSes, it would be advisable to use a drive attached to motherboard for both reliability and stability.
That being said, does NOT mean that you cannot substitute or use USB or memory cards for running (or testing) an OS. If either of these are used, you should understand that you should not try to equate their performance against SSD drives connected internally to your PC.
So, notwithstanding the 'convenience factor'; I would heed the advice you see from others here when attempting to run a "full" (non-RAM) OS from USB sticks or SDcards.
Hope this helps