I think I read somewhere that they coat the hair. I'm not really sure if that's the only reason they are bad. I would like to know the correct answer also.
Silicones cover the hair with a thin film. This film helps the hair to resist humidity (not frizz up so fast), can make the hair silky and easier to comb, and also can reduce moisture loss from the hair. Some silicones build up on the hair though and can be hard to remove with regular shampoo so a clarifying shampoo may be needed. Silicone can make your hair feel really good on initial use but, over time, hair with silicone build up doesn't allow conditioners or treatments to penetrate and can get dry, straw-like and damaged.
Well... You really have to watch your hair and see how it reacts. Some people have found that their hair really doesn't agree with cones at all. I find that as long as I am clarifying and deep conditioning regularly I can get away with products that have some cones in them, like rinse out and leave-in conditioners. I don't use pure silicone serums because I found that these built up and dried out my hair really fast.
If you're natural, cones are not good because the build up requires you to use a harsh clarifying shampoo that contain sulfates, which are really drying to natural hair.