My ex-wife worked for non profit for most of our marriage, these are my summations of her frustrations over the years. She and I both were pretty severely depressed during this period as well, so keep that in mind:
they are run like a business, and can be just as political
you will be overworked and under paid
career prospects are limited
the board is sometimes full of sexist idiots
you will meet some wonderful people but still sometimes work with and for idiots
All that being said you are doing something that typically is doing good for humanity.
I worked for one. Long hours. Pay was probably below average. It was fun, though. Did it for one season in my youth because the opportunity was good for my resume.