The biggest issue I can think of is security, and the questions that you need to consider are:
Does your company/organization have regulatory compliance to follow with regard to data storage/protect (sarbox, hipaa, etc)?
Will you need to separate and manage database access to different administrative users? That is, users in the database software itself, not just people users.
Is multi-tenant use a concern? This is related to the first Q above, but can be a separate issue if you're providing virtual hosting for web sites.
I don't see anything wrong with running multiple web sites off a single database server (though a single database is another story ;-)), as long as you consider the context of the data being stored and the security implications. The logistics of backup, maintenance/management and general administration can be mitigated through various tools automation techniques. Since you're talking about a cloud offering for this (Mosso), then you'll need to consider the performance implications as well.