While Discourse isn't perfect, they have built and matured as a platform, in both polish and reliability. When I get the 'New Version' for Discourse, I am never scared to click that button. I can't say the same about NodeBB. Nodebb still feels like most other open source software, exciting and interesting, but not reliable enough to make me jump in.
Another good comparison in maturity would be Ghost. I started using Ghost and NodeBB at the same time. Polish, feature additions and ease of deployment have all been things you can easily see that Ghost has improved over time. I don't see that same platform maturity with Nodebb.
Not using Ghost so will say nothing, but - for Discourse part - its all way round. Discourse matured over time? Must be kidding! If they wanted to be treated like mature product they wouldn't have implemented three mail providers only, but also all the other mail providers like gmail.....
And per support: Discourse had just banned me because, lo and behold, I voiced my concerns. Oh yea, just for doing so.
So no, Discourse is by no means mature. Whereas NodeBB is.