Vala is an object-oriented programming language with a self-hosting compiler that generates C code and uses the GObject system.
Ranked in these QuestionsQuestion Ranking
Pro Rich GTK Support
Vala developed by Gnome Foundation.
Underneath there is C. It makes some stuff easier to write than it would be with plain C code.
Pro Easy Syntax
Vala is syntactically similar to C# and includes several features such as: anonymous functions, signals, properties, generics, assisted memory management, exception handling, type inference, and foreach statements
Pro Well-integrated with C language
Vala itself is compiled to C, therefore it can use the vast ecosystem of C language, with least effort.
Pro Good documentation
There is Valadoc and most of the libraries have Devhelp packages.
Con Not a general purpose language
Can't use Vala in multiple software development areas.
Con Very limited adoption
Vala isn't used in areas other than GTK development, and no major business has adopted the language.
Con Doesn't have an independent type system
Heavy reliance on GObject types which heavily restricts where Vala can be used.
Con Not well documented
It already has tons of documentation, even a book about Vala.https://leanpub.com/vala https://wiki.gnome.org/Projects/Vala/Documentation
Con A very naïve approach to the real problem
Vala just adds more complexity to the game. No real benefits in using a 'programming language emulator' for UI code, you may think it adds to productivity but it's just a way to make the problem last longer, reinventing the wheel everytime and worse.
Con Difficult to debug programs
Try Debug in Gnome Builder, it works perfectly.
Con Scarce learning resources
Difficult to find good resources for learning Vala.
Con Very few resources allocated to the language
Only a single person is working on Vala full time. Gnome Foundation isn't allocating sufficient resources to properly maintain Vala.
Con No stable version
No stable version of Vala released for over a decade so far. Means that Vala isn't production ready.
Con Very limited tooling
Though using Gnome Builder, you can debug it easily.