If you have thermal I think NV is really optional, but the one thing that NV can do better than thermal is provide depth perception, which can translate into range estimation or at least the ability to differentiate distances.
For example, I was hunting with a bud last weekend and I was describing the terrain as I remembered it and as I could see it wearing PVS 14's. I kept proposing a plan based the description of a doughnut shaped field, with a long thin tree line in the center. With his 640 thermal spotter he could not distinguish the closer centered tree line from the tree line on the opposite side of the doughnut and didn't understand my proposed plan. He grabbed an Envis and within a few seconds of scanning the field, he got it.
As far as ID'ing with thermal, I agree with Lancer, Brian and Oso Grande. Lots of variables come into play. As mentioned, being able to gauge distance really helps. Whether the critter is standing, bedded, head up, feeding with head down, standing on a billiard table field or standing in knee deep winter wheat or grass... Just a few of the variables... But watching from a distance usually removes a lot of uncertainty.
JPK