One problem is that quite a lot of the light produced in the LED does not escape -it is absorbed in the LED and the energy transfers to heat. Also, the more current we pass through an LED (to make it brighter), the less efficient it becomes. In some cases this happens because of the heat produced. But, incredibly, Physicists can't yet explain why in all cases! The problem has been named: LED droop.
One solution to this in a bulb is to have more than one LED. Each LED can be made to be less hot and suffer less droop. This is what one UK company, Totalight has done to make an effective equivalent to the common 50 W halogen - they use three LEDs in a single lamp. Furthermore, they have designed electronics that allow the lamp to work on dimmable circuits - brilliant!
We have tested their lamps for over 6 months and are happy with their performance. There are some differences in size and behaviour, that are worth mentioning - mainly because of the underlying GCSE Physics.