Just to clarify, I don't use Drone Deploy, so I can't really comment too specifically about their 'conversions' or what they might be doing differently now than when I first looked into this. And I'm not 'anti- Drone Deploy'.. I just found Pix4D to be best for my purposes.
There was a recently published white paper that looked at three of the algorithms for plant health indices based upon RGB and compared it to true NDVI (McKinnon & Hoff 2017; Agribotix white paper) and it was not a favorable comparison. That was also the general consensus I got from geographers who routinely use remote sensing data.
But, NDVI (as you may already know) is simply NDVI = (NIR - RED)/(NIR + RED). So if you don't have NIR data (which RGB cameras filter out) then you don't have a true NDVI. Thus, people develop algorithms using just the data available in RGB images to try to do as best as they can for a vegetation index with the implicit understanding that NDVI is better. So far, this hasn't been too successful (McKinnon & Hoff 2017), but that may well improve.
Two of those RGB-based indices are VARI and TGI. And while they seem to do a good job within the purposes of their original development (VARI for leaf-area index and TGI for chlorophyll(and nitrogen)), neither seem to be a good general index of vegetation health and part of the issue is that ambient light isn't controlled very well with strictly (uncorrected) RGB images from (unmodified) consumer level drones.
Now, finally to get to your actual question... if you examine VARI and TGI with an eye toward 'plant health' and compare those directly to the RGB image itself it seems to me that the RGB image is giving you the same info. That is to say, do you really even need an index if you are just looking for dead or defoliated spots? And either way, you'd still have to do 'ground truthing' to find out why those trees are dead/dying.
I hope I've answered your question.. if not I'm happy to try again or dig into this a little more if I don't have an answer myself. I'm a biologist employing some tools developed for/by other sorts of sciency folks.. so I'm still learning too.