1470 open issues
the first one is about phantomjs crashing and many many other scary sounding issues you wouldn't have if you've used just a regular browser with Xvfb.
you can't really be sure unless you've literally run a test on the colorful variety of websites in the wild. You need a huge development and quality assurance team. You also need a way for a large crowd of people to test your browser and send back useful reports automatically whenever it crashes.