He might exaggerate for sake of effect, but I'm not aware of an instance of him getting his facts wrong. In any case, he wasn't responsible for the primary research here, he was repeating things that other authoritative sources had produced. In the case of the deliberate suffering, there seem to be two main damning reports, the first from the Editor of The Lancet in 1991 (this is a widely respected, peer reviewed UK medical journal) - he had visited her missions personally and questioned the way that the care was being given; and the second from the Universities of Montreal/Ottawa in the Canadian journal Studies In Religion in 2013. The latter reviewed 500 separate reports and documents on Mother Teresa and was published as "The Dark Side of Mother Teresa". When I was growing up the general opinion was that she was an unquestioned force for good, but then, people used to say that about Jimmy Savile as well... I think at the very least she was a questionable individual who was good at turning the media story her way. Hitch may have been a polemicist who liked being controversial, but that doesn't make him wrong.
Personally I think the whole concept of sainthood is a ridiculous medieval holdover, which the Vatican uses as a political tool, but hey.