Bart Ehrman claims the reliability of the New Testament is in question because we don't have the original manuscripts. We have "copies of copies of copies" that are untrustworthy because they've been changed so that we can't know what the originals actually said.
It's true we don't have the original manuscripts, but rather copies a few generations down from the originals. But it isn't a sound conclusion from this fact that we can't know what the original said and it would have been better if we did have the "autographs" - the documents the Apostles actually wrote. In fact, it's a good thing we don't have the originals and we can have more confidence in determining the accuracy of the text without them because of the many copies we have. Greg explains here in "God Doesn't Give Autographs."
The fact that we have multiple copies rather than just one original is far greater help to determine later additions and changes. The many thousands of copies we have to compare is more help than one original that may have been tampered with and we'd never be able to tell. That the many copies we have helps us to see that Mark 16 was a later addition doesn't call into question the entire New Testament, it actually gives us confidence that we can distinguish these later changes from the similarity and agreement of the vast majority of manuscripts that we have. We wouldn't be able to do that if all we had was the original.
As usual, Ehrman is accurate on the facts - as far as he reports them - but doesn't report all the facts and draws bad conclusions from them.