It’s just a hosted SaaS application. For the end-user in this context, there’s no particular benefit to the cloud platform vis a vis such status beyond normal hosted solutions (well, that’s not technically true but those benefits are behind the scenes and aren’t a major concern for most of their target audience).
They’re useful in a lot of ways for people who just want to get something rolling. But to be honest, if you’re looking for anything more than that the shortcomings really start to show up. For myself, the biggest sticking point is just a lack of control: in order for their business models to be tenable, they’ve got to lock things down pretty tightly in terms of extendability. So with nearly all of them, it’s pretty much a take it or leave it situation.
With EE and other self-hosted solutions, you have that flexibility. Whether you’re dealing with server configurations, add-ons, or whatever have you, that flexibility can mean the difference between meeting a client’s needs and coming up short.