Yes, definitely high humidity, specifically fog, will "white out" thermal. HOWEVER, fog will kill NV as well. Thermal still works well enough in high humidity, but not as well as it does in low humidity. Humidity contains water vapor that had temperature and significant mass compared to air. Depending on your situation, its 99% humidity with no fog and so maybe you only see 300 yards instead of 600. For most hog hunters, that is still quite a distance, though some definitely need more.
Thermal also works less well when everything including the ground and all the vegetation is about the same temperature as the animals. That means everything blends together. You get no visual contrast.
I hunt with a guy that has one of Ident's PVS-14s and I have a FLIR PS32. Where we hunt, there are a lot of dark shapes that could be hogs that are not. Thermal roots out what has body heat and what doesn't. So instead of wasting time to see if the dark blob moves or not, he asks me if it is giving off heat.
The big problem I see with NV in general is that it still relies exclusively on contrast, which is a significant component of human sight. Sure, seeing black dots in a light colored field makes NV look amazing. You move into a shadowy forest environment and that strong contrast disappears. It can also be a problem with simple blending. A brown deer in tall brown grass can blend in very well. That is because there is a lack of contrast between the deer and the background. Part of how you can get over that is with an illuminator. Eyeshine is outstanding for spotting many animals. However, in a highly cluttered environment, using an IR illuminator often results in overillumination of items close to you and underillumination of items beyond, just like when you take a flash picture in the dark of a group of people up close, but all the people in the background are underexposed compared to the people in the foreground.
Since I don't drive around and try to see through my windshield, I have the opposite view of TLM. Thermal is the key for searching out targets in my world and I verify them with NV. In other words, what is necessary wholly depends on your mission parameters.
Oh, and while NV can see through windows and you can correct for some of its shortcomings by using in IR illuminator, you can't expect to shine the IR illuminator through the windows anymore than you would try to shine a flashlight through your windshield to illuminate your way if your headlights don't work. WAAAAY too much light will be reflected back at you. So the IR illumination correction for humidity doesn't if you are in a vehicle with the windows closed very well at all.