For heavy/severe turbulence most small to medium ops manuals say to disconnect the a/p & fly manually. Why? The a/p will follow every movement & try to correct it. Imagine an a/c rolling & pitching at the same time & the a/p followng all these to correct it. As Makii152 says, you could finish up in a stall. The idea is to allow the a/c to ride the turbulence & you to keep it under control by not following every movement but to manually keep the a/c in a safe and controllable situation. The 747 classic a/p has a turb mode selection that halves the a/p inputs and that is na a/c that is pretty resistant to turbulence.