I could but… the bottom line is: it’s a shiny turd.
Very simple to get in to, very hard to make something complex and nicely working and polished code-wise. Nearly impossible to do good QA / tests / debugging unless you really know everything about the inner workings.
Basically the amount of work to create something good/complex in Unity, is as much as with a professional engine, I guess even more. But this work would be extremely frustrating kind of work.
If you want to make something very simple, very fast and it can have bugs (basically a prototype), then you may use Unity. In other cases building up expertise with any other engine/framework will be a better use of time.
The knowledge you gain from Unity is useless and specific to Unity’s behavior, not all of it, but major frustrating parts. If you learn libgdx, virtually all these concepts can be found in other big engines as well and will help if you have to work with something else too.
Well hey - if I wouldn’t know that feeling, I would be a C++ game programmer by now I guess