Ranked in these QuestionsQuestion Ranking
Pro Under GNU GPL
A better license for a browser than a restrictive "all rights reserved".
Pro Open source
Anyone can contribute to the Otter Browser, solve common issues, bugs.
Pro Legacy Opera
Aims to recreate the best aspects of Opera 12.x.
Con External Rendering Engine
It uses the Qt Web Engine from your local Qt installation which is usually not often updated.
Con No support for addons
Con Under GNU GPL
It is licensed under GPL which is less permissive than other opensource licenses like BSD or MIT.