Some of the issues I see in one of my projects is high interactivity between accounts. E.g. if account 1 'sends' something to account 2 both of the shared/separate db instances need to be up or there'll need to be some kind of queueing mechanism.
That's hard enough and then add to it that most clients want to BYOK to those instances
High interactivity between accounts is a good reason to not adopt the proposed multi-single-tenant architecture. The scenarios discussed are B2B enterprisey apps in which the environments are essentially independent from each other.
That's hard enough and then add to it that most clients want to BYOK to those instances