Both the Apple Pro Display XDR and LG 5K display are connected via Thunderbolt 3. This allows these displays to provide three (3) high-speed USB-C ports, which is terrific as these are very useful to my work, and there are only two (2) USB-A ports on a Mac Pro. The Thunderbolt 3 ports on the Apple video cards also can be used as USB-C ports.
Both solutions SUCK. Why? because both video cards and displays sometimes (not always) cut connectivity to those ports when the system comes out of sleep, which hard-disconnects any attached peripherals, such as hard drives or SSDs.
I don’t know if this behavior is hardware or software or both , but it wasn’t a problem until about late 2018, which means either the latest Thunderbolt 3 shitsets (ooops, chipsets), or macOS Mojave and macOS Crapalina.
It’s bad news for attached storage. Hopefully all important data is flushed to disk before the Mac enters sleeps, but if a program has an open file, disconnection of the drive kills the open file and that could result in serious problems like data corruption. Fortunately, most programs do not maintain files in an open state, but some do, including the worst case of databases, which have complex structures and must not become inconsistent.
And of course, booting off an external drive connected through a video card or display is an automatic hard crash when the drive disconnects in this scenario.
This is very poor design at all levels—more egregious Apple Core Rot.