Sulphur lamps are less efficient than the best HID lamps, comparable to the best fluorescents. This is mainly due to having to excite the sulphur using microwaves. The actual emission process is as efficient as an HPS lamp, but there is a further 30% loss in producing the microwaves. So no real drive for anyone to want to use these lamps. I don't think they are even available commercially any more.

A few more problems for plant growers. It is impractical to make them in low powers. The Philips lamps that used to be sold was 130,000 lumens and that is about the minimum possible from one lamp.

The spectrum is continuous, but it is strongly peaked in the green, with little blue light and very little red light, so PAR efficiencies are quite low. There have been some experiments with additives to produce extra red light.

Maximum bulb life is extremely long, but the microwave generators only last for 10,000-15,000 hours. Add in motors to rotate the bulb and fans to keep it at the right temperature and the whole thing gets a little unreliable.