Is Robinhood safe to use?

Yes, it is, until certain bounds. I will list some reasons:

1-Even they charge you nothing for trading transactions they have a business model that follows a similar model with that of banks when it comes to monetization. Like banks, they collect the customer's assets then they have the ability to lend these assets out and charge interest on it.

2-They seem to be reliable and competent people
One of the founders has a PhD in mathematics from UCLA, while the other has been working in finance. It seems a good combination

3-They are backed by some good investors
such as Google Ventures, Andreessen Horowitz, Aaron Levie, Snoop Dogg and other companies.

4- Great app and already have a large number of users in just two years : 500 million dollars transacted so far on platform

This post is also my Quora answer here https://www.quora.com/Is-Robinhood-safe-to-use/answer/Alket-Cecaj?srid=n9bS