Her strongest critic was Christopher Hitchens who has a tendency towards overstating the case he has against any particular person. To save myself some time I'll just copy an old comment
I've read The Missionary Position. As an argument against canonization, I suppose it makes its point, in a rather roundabout way. As an argument to the end that she was actually a force for evil, I found the book terribly unconvincing.
For one thing, Hitchens often appears to be accusing her of not matching the standard for what he thinks her mission should have been. The House of the Dying was set up as a place where the poorest of the terminally ill in Bombay could go to die in a space that was calm and clean. Faulting it for not living up to the standards of a hospice in a Western country seems to miss the point.
Even if you can make an argument that the House of the Dying is somehow less than moral or less than humane, character assassination doesn't really do anything to address that problem. Hitchens would have done more for the "victims" of the House -- if a voluntary admission, who would otherwise have died the same death out in the streets, can really be called a victim -- if he had instead argued for the establishment of an alternative. If I'm not making myself clear, saying that Teresa was no saint does nothing to actually change the situation of the dying; setting up a rival House of the Dying that does offer morphine would.
For another thing, Hitchens comes up dreadfully short on citations. I'm a bit of a citation freak -- if a claim in a non-fiction book strikes me as interesting, weird, dubious or otherwise worthy of looking into, I'll mark it down and look it up later on, if possible. For the vast majority of claims made about Mother Teresa by Hitchens, that's just not possible. And that's a serious liability when you're attempting to assess any argument that's loaded with so many rhetorical flourishes and hyperbole. So, for example, when Hitchens says
For example, when there was the Second Vatican Council, in the 1960s, there was an equivalent meeting for the Catholics of the Indian subcontinent in Bombay. Mother Teresa turned up and said she was absolutely against any reconsideration of doctrine...
we are, presumably, just supposed to take his word that that's how it actually happened. I don't know of any particular way to fact check him on that. I don't even know how he knows it. Was he there? Did he read it in a report? Was it related to him by someone who was? How many messengers stand between us and the actual event, and how many of those messengers are reliable? And if we're not sure about those things, then can we be sure that Mother Teresa really came in an made a determined rejection of any reconsideration? Maybe there was more give and take than that. Or maybe there wasn't. I'm just not sure why we should take Hitchens' word for it.
And lastly, I think some of Hitchens' objections against her are just down right petty. Sometimes it's guilt by association -- he's not terribly fond of the Clintons, so he dwells on their association with Teresa as an index of her duplicity. Other times it's wildly more interpretive. He's fond of that anecdote about her arriving at the Vatican by bus. "A normal person would put on at least her best scarf and take a taxi." Really? That's his objection? That she isn't a "normal person." Because, presumably, normal people don't take the bus or wear inexpensive clothing. Hitchens could have interpreted that in all sorts of ways. As a former socialist, he could have seen in it a resolve to not cover up the humbleness of the culture that Teresa worked in. It seems clear to me that he's interpreted it in the worst possible light -- and really, not spending donations on a taxi in Rome is supposed to be a bad thing? -- because he wants to see it in the worst possible light. And if he's willing to put that much spin on something as innocent as that... well, it's all the more reason to wish that he had provided more citations for his other claims.
None of which is to say that think Teresa is the saint she's made out to be. I don't necessarily agree with her positions on most issues. I think that her behavior was often a bit contrived, and there probably is a strong argument to be made to the effect that she was psychologically morbid. If the allegations about her mismanagement of donations are true, then I'd like to see it proven. The point is that Hitchens doesn't prove it. He's content to simply repeat the allegations, often without attribution, and to suggest that these things add up to the inversion of good.
A better case needs to be made.