I am booting several of my RPi 3B and 3B+ from USB mini thumbdrives*, and this works very well. The speed increase compared to my micro SD cards are in the 2-3x range, and I don’t have to worry about the SD cards corrupting over time.
The only problem I see, is that the USB thumbdrives and the USB ports on the RPi run very hot. I always plug the thumbdrives into the top ports to keep the heat as far away from the curcuit board as I can, but still I fear that the heat may somehow damage the RPi itself. The ports get so hot that I can’t stand to put my finger on them for any length of time.
I have considered using USB extension cords to move the heat further away from the RPi, but that defeats the purpose of using as small thumbdrives drives as possible to get the RPis as «clean» and elegant as possible.
What do the experts say? Will the heat be something that I have to get rid of, in order to use the RPis without risk of damage?
* The Sandisk Ultra Fit 64GB and the Samsung Flash Drive FIT 64 GB