Despite being someone who has used a single USB SSD for both boot pool and apps pool, thus breaking not one but two of ixSystems strong recommendations, I fully stand behind this statement.
My use is for a home media / backup server, and high-availability is not a requirement, but cost very much was a concern. So I took the decision to do this with the explicit knowledge that this was not a supported option.
So far, this decision has been OK - aside from a glitch where the GPT Primary Partition Table became corrupted (which could have been a USB thing), this has worked just fine. I have upgraded SCALE versions twice without any issues.