FD below minima are a useful tool by providing pitch guidance/crosschecking during marginal weather or weather/terrain/runway light conditions that may bring visual illusions.
However,over reliance on FD by flightcrews,especially below minima,may lead crews to lose their visual cues(maintaning a constant descent and aiming point) which could lead to unsafe approaches when FD are not providing proper guidance and/or switched off.
I believe in switching off/on FD as a general rule when descending below MDA unless as stated above,during few exceptions.