Oh my, yes.
And your surmise about how prosecutors deal with "iffy" cases is correct. If the witness is going to "flake out", the case is going to go south, so there's usually an effort make sure the witness knows what she's about to face at trial and to be sure she wants to go through with it. This is sometimes interpreted as an effort to discourage the witness, but it's really just exposing her to reality and making sure the people's money will be well spent.
With cases with slam-dunk facts, (public, injured, multiple witnesses, etc.) it's a different story.