Saying they don't work and can't contain enough sugar, is like saying that a sugar lump isn't the same as one teaspoon of sugar and won't work for carbonation!
Carb drops are made of sugar (sucrose) and glucose. They are pretty much the same as dropping a sugar lump (cube) into your bottles, except sugar lumps are pure sucrose. Glucose is actually easier for yeast to "eat" than sucrose. I suspect that the drops are made in liquid form then pressed into shape for packaging.
They were originally made for the Australian market, where they use smaller bottles, called stubbies, which are 375ml. Therefore, one drop is not enough for a 500ml bottle, you need one and a half. A great many people use them without any issue, although most people I know tend to chuck two drops into a bottle, rather than cut them in half and add one and half drops.
They are expensive compared to using sugar and a measuring spoon, but for many people, using a measuring spoon and trying to get the right amount of sugar into 40-odd bottles is far more hassle then using carbonation drops. A bag of drops is 250g and has, I believe, 80 drops, making each one 3.125g. A teaspoon of granulated sugar weighs approx 4g. When I batch prime, I work to around 3.5g of sugar for an ale, as I don't want them too fizzy, but some people use more.
Carbonation drops will give you carbonation, even at one per 500ml, but it will be on the low side. If you're not getting carbonation, then your bottles are probably too cold.
Batch priming is, however, probably the best way of ensuring even carbonation across all your bottles, and is the method I use.