Craft can handle everything you have described with one caveat:

The "few discussion boards."

If it is a very basic setup then you could build something. For example, if it was similar to a comment system, such as the one I'm currently writing on (here at Hacker News), then I think it could be done easily with a comment plugin:

https://github.com/engram-design/Comments (note the threaded commenting and that by adding a few other plugins you could make it more feature rich than even this. Josh (@crawf) is almost always in the Slack channel if you have any questions about any of his plugins.)

If you need something more advanced, with allot more features etc, you'd currently want to look into third party options (since you don't have the time and or resources to build a plugin, I'm sure and one it's currently available.) For example:

Using a third party forum system like Vanilla and setting up SSO via a Craft plugin: https://github.com/nystudio107/vanillaforums

Or a discussion system as a service like: https://muut.com

Craft is new compared to some though, it is going on 4 years old (https://craftcms.com/changelog#build2063). Pixel & Tonic, the company behind Craft, is definitely not new, however. They've been around for along time and built some amazing add-ons for ExpressionEngine before they started Craft. The Matrix being one (https://craftcms.com/features/matrix)

Are you still worried about it being new? Well, consider a different perspective in that this is actually a good thing. That is, it doesn't have all the baggage and bloat those other systems continue to carry. They are built upon a different paradigm and are married to said paradigm (religiously) because it is what they started with, it is what they know, it was what has fed them etc. and what they continue to know and understand.

With Craft, you get a modern system which is very user friendly and, at the same time, as powerful as you'd need it. And yes, it is "flexible and lean" because of this. You can just "feel" it (the others "feel" heavy.) The UX/UI really is amazing!

Craft's source is open and built using OSS but, it's not FreeOSS. I don't want to get into some sort of debate about what that means here (if you're an org that needs FOSS then you need FOSS; don't use Craft) but, I would like to point out that paying a mere $299 buys me some of the best support in the business. Really, their support is stellar! Often times I speak directly with Brandon or Brad and many other team members when I have a question or need help (via Slack or Twitter or Email etc.) Good luck finding this with any other CMS for 300 bucks.

The license is pretty basic and is really in place so they have a business model to continue to build Craft itself and this long into the future. I like that. I, for one, don't mind supporting them. I see it as a win win (I think I'm getting the better end of the stick I might add.)

If you have any other questions feel free to head over to Craft's Slack channel (https://craftcms.com/community) where you'll find allot of community members who are more than happy to answer any of your questions; when and if they arise.

Continue your research and good on ya for doing your due diligence. Oh, and good luck with the project!