Games have been made in Unity at all levels of the Video Game industry. If you want to make games, this is a great engine to learn. Learning Unity will teach you the basics for any engine, and if you want to get a job at a big studio there is a chance that you will be working in this engine there...
The editor GUI is very powerful and intuitive. It allows pausing gameplay and manipulating the scene at any time as well as progress gameplay frame by frame. It also has powerful asset management and attribute inspection. This allows it to be more powerful than other, simpler drag-and-drop engines
Performance could be real issues with 2D games. Unity 3D is actually 3D game engine, and drags a lot of overhead with it. For mobile, one could never achieve performance of more specialized 2D engines with Unity3D.
This engine is not well put together. Is made from various free modules each with their own peculiarities. At times it feels you need to learn a couple of libraries rather than just one. Is not an engine for beginners as it requires coding. Lots of coding. You need to be intermediate to advanced in
Other than a brief installation / getting started overview, libGDX's documentation consists of an official wiki with several incomplete pages, and automated Javadocs. The community recognizes these shortcomings, and new users are encouraged to ask for help.