Twig is fully documented, with a dedicated online book, and of course, full API documentation.
Twig is flexible enough for all your needs, even the most complex ones. Thanks to an open architecture, you can implement your language constructs (tags, filters, functions, and even operators) to create your very own DSL.
When it comes to security, Twig has some unique features.
Twig is fully unit-tested. The library is stable and ready to be used in large projects.