I'm using Mithril because of how easy it is to look over the source. I didn't want to have another framework with a huge learning curve (even for senior devs) like Angular. It's well documented, and you can read the entire Mithril source and understand what it's doing within a few hours. That means I don't have to bother training people on it. I can give any new people the API and a link to the source and they should be able to contribute within days.