This and most everything else you'd want to know is addressed in their official Unsplash License guide and FAQ: https://help.unsplash.com/en/articles/2612329-releases-and-t...

> While photographers agree that photos they upload on Unsplash have model releases, there is no reasonable way for us to monitor all photos that get uploaded to Unsplash. We cannot make any guarantees, therefore we recommend when using a photo containing a person, that you should not imply endorsement by the person in the photo.

It's a great service but how careful do you need to be with caveats like this compared to other sources?

Unsplashed was initially a hand-picked collection, and as far as I know submission is now heavily curated (and probably checked with reverse image search).