Unfortunately it’s not just an overhead in onboarding, but also a source of potential incompatibility with other libraries.
A recent example:
This issue was also raised on React’s repo, and the note from React’s team member is an indication of how the use of node-fibers is still keeping Meteor from becoming a part of the “mainstream”:
React supports standard-compliant JavaScript environments, so using an environment with non-compliant concurrency primitives is going to cause problems.