At low currents it’s somewhat inefficient, at high it’s very efficient.
Please let me explain…
All LEDs are more efficient at low currents than they are at high.
As you driver them higher, output increases. Similarly and more importantly for comparisons - surface luminosity increases. To reach high luminosity you need to sacrifice high efficiency. Now…some emitters lose efficiency faster than others. White Flat starts not very efficient, but its efficiency drops relatively slowly.
The peak output is achieved at about 50 lm/W. In general - horribly inefficient. But then - no other LED gets the same luminosity with higher efficiency, so is it bad?
Here you can find surface luminosity vs. efficiency chart. White Flat is not there but there are the previous generation emitters. White Flat is more efficient across the range, the curve shape is similarly flat.
So…it’s modestly efficient at low surface luminosities, but compared to other emitters - somewhat inefficient.
At higher luminosities it’s very inefficient but compared to other emitters - very efficient.