My experience with thermals is limited, mostly to Iraq and the Mojave, which are deserts and not the best usage cases during the day (deserts are hot, yo). I didn't not play with individual thermals in AFG, only the ones on my turret.
Anyway... unless they have improved remarkably (and the newest ones I saw in the Mojave were very impressive at night, so they might have since I retired) they are not the I win button they are here for various reasons. They seem to have been toned down in game from what I saw in videos of them in beta, but.... Thermals, especially during the day can get a lot of false positives and a lot of 'background noise' from fires, vehicle wrecks, concrete, animals, etc. All kinds of things mess with ambient temperatures and they can be blinded by other heat sources, similar to how bright light sources can blind NVGs. During daytime, especially hot days, the ambient temp makes the temperature differential much closer which makes it more 'washed out' looking and limits range and clarity.
Currently, as they are in game if you see any heat source you can almost be sure its a player and with a quick hover over it the spotting mechanic confirms. So, without intervening terrain or ghillies they are pretty much impossible to counter. The spotting mechanic, in general, is cool but works way to well with just sweeping with your lens until you see a blip, backtrack to hover over it and there ya go. Spots. The big advantage in game of thermals is I can skip the scanning process usually and just see by contrast where people are where at least with the rest of the recon kit I have to do the sweep and scan.
Also, seeing even more cheaters. Most obvious one was last night, guy was 21-1-1 and was just running around assaulting with an SVD. No assists other than that one, so not spotting or anything. Examples of stuff like at least 4 head shots on 3 people in a Kodiak in less than a second, while moving. I know he hit me twice and my gunner and passenger were both dead before we could tell each other over discord. Timestamps were near instantaneous. Same dude was running up the stairs of a tower while I was hiding 200m back in bushes. I fired into a different tower only to get double tapped on him as he's running up the tower stairs. Needless to say we reported him.
Its really getting bad, and I know people say its Early Access and they are letting the cheaters run to gather info to build better counters, but lets be honest. The only thing early access is their access to my wallet before calling it an actual launch. They need to do something about the cheaters or at least acknowledge them and give some sort of plan and promise, otherwise this game is going to start to suffer. Once again, I'm eyeballing community run servers as the way this game survives.