Just thinking now, a lot of the legendary projects came from great MC's paired with great producers (duh). But when the MC's left to find their sound on their own, their work seemed to suffer.
The legends who bounced around from producer to producer had similar problems. Nas, early Jay, later Rakim, later Eminem, later KRS, and Pac suffered from the consistency issue. That may be a big reason for the lack of consistency in quality, album to album.