A common and basic criteria for any resource, educational or not, open or not, is correctness. Instructional material, no matter how well written, well designed, or well marketed, is worse than useless if it is not correct. A messaging utility is a poor investment if it loses data or misdirects messages, no matter the cost.
Open source materials have a clear advantage here. The inclusive, community driven, nature of open source makes it more likely that the community will provide constructive feedback. A well organized open source project will invite the community to contribute updates and corrections directly. Such constructive community involvement is important for the health of open source projects.