Well a web app is a user interface. Obviously there are backend components to most web apps, but any of the client-side work can still reasonably be referred to as UI work. You just need to specify between UI designer and UI engineer.
Yeah and using UI Engineer as an alias for frontend developer is fine. But the term "web developer" is a more broad term that covers front and back end work, so I think it's a bit weird to start referring to all web developers as UI engineers.
