User-Friendly Interface, very easy to learn for beginners.
Supports multiple development platforms , mobile, desktop, web and console.
Deployment to multiple platforms is very easy to manage.
Unity has its Asset store where you can find/buy stuff, that you want to use in your games.
There is a lot of free stuff too.
Developers can easily afford its paid license.
Heavy engine.It takes lots of space in hard disk for installation.
Windows-only. No editor for Linux.
Expensive if you need all features.
Working on big games takes a lot of optimizations.
The engine source code is not available. The paid users also can't see its source code.So,it is difficult
to change the code,if in meanwhile we encounter any bug.
This also limits the ways in which you can extend or customise the engine.