Much already have been said about it, so i just leave some comments on each thing, and some links with all the source.
Bug reports corcerning these “missing features” of the Su-39.
SU-25TM(SU-39) missing KHOD FLIR pod // Gaijin.net // Issues
Su-39 Missing “Khod/Kinzhal” pod (FLIR+RADAR) // Gaijin.net // Issues
Su-39 Missing “Kinzhal” MMW // Gaijin.net // Issues
Visual confirmations of the khod container and a scheme:
Spoiler 1
There is also a third pod that was datamined and till this day it still in slumber. I don’t even know it would hold, but my guess is if the Mercury and Khod used the same container, this one was used for testing the Kinzhal-S on the T8TM-2.
Three containers below:
Containers
by Overscan
"This Mercuriiy pod was displayed on the Su-25TM - the shutter covers the sensitive optics. Whether it was functional, I don’t know. The results of testing were not good. It was intended to be replaced by a Khod FLIR in the same container.
This is the Mercuriiy for the Su-25 below. It has the fixed circular wide-angle view lens as on the V-80 prototype and a separate and a narrow-view lens that is moveable. Source: Ildar Bedretdinov book on Su-25"
source:
Soviet targeting pods (Mercuriy LLTV) | Secret Projects Forum
And Gaijin judges it as such, because in game Mercury uses the same container as Khod.
Su-39 suffers immensily with any bad weather. Rain and clouds makes you incapable of engaging any targets far than 7Km. The only USSR aircraft that have a thermal imager is the Su-25SM3, and the Su-34 has the Kh-38MT, which the Su-34 can use the thermal seekers on the missiles to see.
Hardly you would find NATO aircraft at this 11.3 lacking a thermal imager – even the Chinese got thermal imagers
The Khod FLIR would be essential, but having also the possibility of Kinzhal-S would be very interesting because it also is able to guide the Vikhrs missiles, just as the kopyo-25 does, but being able to track static objects. Thus the vikhr could be a very effective weapon for use with the mmW radar, in a very unique play style.