Unity 3D vs Mobile / Web App

I am doing research for a gamified fitness project and I am trying to decide whether to develop it in Unity or as a mobile/web app.

I am aware of the many things that Unity can do that mobile apps can’t, but are there specific things that a mobile/web apps can do that Unity can’t?

Can a project developed in Unity have the same functionality as a mobile/web app? If not, what are the key differences and limitations?

Any thought or insights about this would be greatly appreciated.

If anyone is curious about this, I found these threads and articles that answered most of my questions:

http://www.jarcas.com/studios/?p=208

Any other insights are welcome.