... Every light puts out certain PUR. (photosynthetic usable radiation) The largest drawback with LED from my experience is the PAR rating. This is a measurement of photosynthetic usable radiation at X distance away from the light. The X is where lumen output comes into play. As for now, LED looses it's charm for most at this point. To achieve the same footprint of PAR that say a 1000watt metal halide lamp throws out you would need approx. 6 400 $ LED fixtures.
There is a huge can of worms I could open here about heat,energy and the rest...