Some have speculated that one of the benefits of LED lights

Some have speculated that one of the benefits of LED lights may also be a drawback:  case in point is the fact that LED grow lights run very cool and produce very little excess heat, unlike other lamps such as HPS lights and MH lights.  It has been said that plants grow leggier under LED grow lights than other grow lamps such as HPS lights.  Light is not the only item that plants need for proper growth.  Bottom heat is needed to get seeds to germinate, and warm conditions are usually optimal for fast plant growth.  Since other grow lights such as 1000 HPS lights put off more heat, it can contribute to warmer growing conditions.  This may mean that a heat source might be needed for use with LED grow lamps to bring on equal growth results.1 

Another area that some claim as controversial is that additional blue spectrum is needed for stocky stem growth in plants than is given by LED commercial lighting.  NASA has performed some testing by its Astroculture Flight Unit, where they determined that LED lamps with eighty percent red spectrum and twenty percent blue spectrum produced better growth than HPS lights and other HID lights, yet neither produced better growth than sunlight.2   If you want to boost the bloom cycle, you may choose to add additional blue spectrum LED lights to mimic summer sunlight and the resultant productivity of plants in response.  Some of the latest LEDs also allow you to adjust the amount of each spectrum, so you can adjust them for optimal plant growth.