I think there is a problem detecting targets, but not with eyes only but with the TGP in WHOT or BHOT. The terrain in all sims are rather flat, even in DCS. In the real world there is trees, bushes, things, alot of places to hide in, which are hard to simulate with today´s graphics. So it should be hard to detect targets, but maybe not this hard. Look at these two images, do you see the tank? Normal Gain settings.
https://rymden.files.wordpress.com/2016/02/screen_160201_194744.jpg
https://rymden.files.wordpress.com/2016/02/screen_160201_194750.jpg
I think ED should at least make the flir better so the tanks really shines. (I have no experience with Flirs in real life, so i don´t know if it is realistic or not.) However, in almost all other sims out there, the targets in the Flir stands out (for example in the old classic Longbow 2, in Steel Beasts, Falcon and so on)
Btw I'm new to this forum :)