I think what scares a lot of people is the prospect of maintaining a server, configuring it to be secure and keeping it up to speed with security updates. With a cloud product, the only concern becomes keeping your project's dependencies updated which is less intimidating.

It's something that's on my mind when I think about launching a site that's intended to draw a significant userbase. Back in the day I'd set up VPS instances with nginx+unicorn+rails and it was relatively smooth, but security has seemingly become so much more critical that I don't know I'd trust myself to get all the biggest holes patched up and more importantly, keep them patched.