However this bomb wasn't used before June '44, when most of the death camps were closed or closing, and the short supply of Tallboys meant that they were exclusively used for targets of high military importance.
As for the targeting: you have to go really low. British bombers attacked the gestapo hq in Copenhagen by going so low that their bombs were dropped into the sides of the building (consequently, although that wasn't the plan, a number of prisoners who had been housed in cells in the top floor managed to escape).
Of course you can only do something like that if you are willing to paint a huge target on yourself because you will be very easy to shoot down.
That's a bit of an exceptional case, but still, it isn't trivial.
Low and fast was safer in many scenarios, but it wasn't without drawbacks.
Anyway, what I've usually heard is that they should have bombed the camps themselves, not the rail lines leading to them. That's not what I personally think, but that's what I've heard others talk about.
Now something like Treblinka was purely a "killing factory" with no prisoners held for more than a few hours before they were killed. In that case bombing everything would probably have saved lives in retrospect, since any prisoners at the site would have been killed anyway, and destroying the facilities would have slowed down the extermination project. But while the allied knew mass killings was taking place, they didn't know enough about the locations and logistics to make that call.