I volunteered for three years in a student NGO. More than a pass the time to feel good about myself, it was an enriching human experience I'd recommend to everyone.
Working with others, of my own free will, taught me a lot about myself and how powerful collaboration is. It's not the same as working in a company for a monthly paycheck. You have to be self-motivated to get started, but the work and the friendships you develop will make you come back for more.
However, the strength of an NGO is also the source of its problems.
An NGO is often synonymous with a non-for-profit organization, whose economic model is based on free labor (volunteering), donations, public funding, and private sponsorships, in exchange for work benefiting society at large.
A bad NGO is quite similar to a bad startup: guilt-tripping the labor force into more work to receive more fundings, no self-sustainable economic model, and too much centralization.
You would be surprised by all the political drama and the bureaucracy happening in a non-for-profit. Some decisions take years to be implemented because of petty political interests and mind-numbing management processes.
The problem is that the world is changing fast, and an NGO would benefit a lot from increasing its iteration speed while developing more flexibility. That's one thing a good startup is capable of.
I'm not sure what an NGO-Startup hybrid would look like exactly, but I think it needs to be self-sustainable and always self-optimizing. We need NGOs and more volunteers to make a change, but we probably don't need more NGOs doing the same things. We don't need more worthless startups either, but in both cases, failing and experimenting over and over will bring us closer to what we need. We should instead try out new models that cannot be labeled as either startups or NGOs - new movements, communities, and systems, with the people's interest at heart.