OK, I found this on page 67 of http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-host-management-guide.pdf
"Use the embedded database. This option is available only for a small
inventory size, with fewer than 5 hosts and 50 virtual machines."
That's exactly what they said in 5.0 too. I tested this on VCSA 5.0 and indeed, bad stuff did happen. But it was all related to DB2 and the horrible default config settings VMware shipped it with. I have a pretty good hunch that the Postgres installation in VCSA 5.1 will work much better in this regard, and VMware is just erring on the side of caution by leaving that disclaimer in. I'll be stress testing this over the next week.