While I’m not particularly invested in their choice, I will say that I’ve got some counters to the points given as to why not:

• Logging for diagnostics: probably the closest point, but you can either centralize such logs where local disk does not matter, or leave log in ram with aggressive rotation out.
• Ability to update without rebooting. The diskless systems I work with can be updated live too. However live updates do eat more memory in my case due to reasons that will be clear soon. Besides, a rolling reboot should be fairly non disruptive to “bake” the live updates into the efficient form. Other Diskless situations just live in tmpfs, in which case live updates are no problem at all, though it is a lot of ram to do this.
• Diskless uses too much RAM: At least with the setups I work with, the Diskless ram usage is small, as the root filesystem is downloaded on demand with a write overlay in zram to compress all writes. Effectively like a livecd generally boots, but replace cd with a network filesystem.