Pros
Colleagues mostly fun and great to work with (depends on the dpt. you enter), salary competitive, good perks (food, drinks, social events), some of the project work is meaningful and interesting. Also a decent amount of travel, depending on your specialization and specific projects. Few opportunities to climb the ladder.
Cons
CEO tries to control everything, despite promulgating "flat hierarchies". Projects you manage have to align with CEOs demands, which are often silly and do not align with foundation's overall strategy. Administrative tasks in project management far outweigh interesting, content-oriented work. You will often feel more like a bureaucrat who needs to ensure that every penny of every project is spend accordingly, creating budgets and dealing with general paperwork (mails, internal forms, drafting notes, papers etc.). Disenchants non-profit work quickly. Foundation does not live up to its hands-on, enterpreneurial values.