It's actually interesting because the movies end up good when they're rewritten (CR had Paul Haggis after them and SF had John Logan) and not so much when they rewrite. QoS - a film I love despite its flaws, incidentally- had them rewriting Haggis' infamous draft with James Bond Jr. before the Writer's Strike hit while SPECTRE brought them in to fix Logan's 1st draft only to be eventually rewritten themselves by Jez Butterworth. I would even daresay their rewrite was the best possible version of that film because Butterworth's work (coupled with some not very judicious editing) muddled a few interesting points in their draft, all things considered*.See, Purvis and Wade wrote Casino Royale and Skyfall.
They also wrote Quantum of Solace and Spectre.
So their reliability is straight down the middle. But based on their interesting sequence (good movie followed by bad movie followed by good movie followed by bad movie), maybe Bond 25 ends on a high note.