One can never be sure about such things. If one thing is certain, it’s uncertainty. Since unsplash+ (like unsplash) is a service of Getty images, one might think it’s legally ok.
Yes. There has been previous discussions about using free services (eg unsplash) for images and legal issues later on. When images have been uploaded without permission to these types of sites. There have been several court cases where the original owner has taken legal action against a person who downloaded from said services.
The + subscription seems to offer more protection in this regard.