Haut Tec wrote a great article on this the other day - SaaS: All PaaS are not created equal.
...The marketing for most products described as “PaaS” for SaaS today includes some combination of the infrastructure and technology levels of the stack and quite often impacts the way the application logic is expressed. They rarely address multitenant architecture or any of the SaaS functionality that impacts the operational needs of your service. Depending on how they are configured, they can often represent a certain amount of risk through “lock-in” to their service...
Definitely worth a read.
ZeroTouch IT Ltd