The topic sounds a bit confusing as one could probably think "what's really new?". You're right, booting ESXi from USB or SD card is quite common and no one would ever could think about a situation where this combination could be problematic. In the recent past I kept myself busy with VMware's new storage technology called VSAN. There are several very good blogs out there if you need further information about this technology but for my post, the most important thing to mention is that VSAN has a direct influence on supported boot devices.
To cut right to the chase, booting ESXi from an USB/SD card in combination with activation of VSAN on that system and a memory configuration of >512GB is NOT supported. You need to use a magnetic disk as boot device.
Why do I write a blog post about this? Well, VSAN is getting more and more popular as it prospects a really simple, affordable and scalable storage solution for ESX. As VSAN uses direct-attached harddisks and SSDs one can think about reserving as much HDD slots as possible for the VSAN storage. Using an USB thumbdrive or SD card would free up another (or even two) slots. This can be important on 2U units where you probably have only 8 or 16 slots.
Next point, there is nothing more important on any hypervisor server as RAM. 512GB is a big amount but with todays RAM prices dropping it's not unusal and is getting more and more mainstream.
So it's quite possible that you will run on this (often underestimated) problem. The reason for this unsupported config is that VSAN will dump core traces on disk and with configurations >512GB there is too much load during dump process to accept USB/SD cards.