I have a fair bit of thermal experience, i have 3 and have used them for perhaps 15 years as a work tool, before stalking style ones appeared. They need technique to get the best of them. Basically you wont see a deer shape in the distance, if you do you would have seen it easily with the binoculars. Firstly lose as much sky as possible from the image, they work on temperature differential and the sky is so cold a slightly warmer animal will not show nearly as well when you have a lot of sky in the image.
When using mine i just look for any heat signal, and then check out what it is. In some areas they simply wont work for hot rocks and branches, and in these situations i simply use the binoculars. In the past i have been looking at a couple of pixels of heat, only to spend ages with the binos before seeing a roe ear sticking out of the heather. i now never ignore any heat, and it can often be a grouse head or rabbit. My pal Bruce built me a stunning roof mounted remote one, it will see a roe at 1000 meters and more easily, we view it on a screen on the argo. It is not so much of a stalking tool as a spotting tool, and we can survey a whole valley with it, count animals and plan routes through the valley so as to not spook animals, whilst heading towards a chosen target. The smaller hand held units will see roe sized targets at 800 ish meters, perhaps a little more on a cold day, reds can be seen at about 1000 meters if in a herd, single animals are less easy to see.
The published figures for thermals always underestimate the range we can see animals in scotlands cold temperatures, but they are for a man sized target, so a large red deer is way easier to see. With roe often you can only see a head in long grass and the like, so ranges are reduced, but these are the deer the thermal sees way before an expert with binoculars. What is certainly true is that owning a thermal will not make a beginner an expert, and they will probably not see any more deer unless they set it up properly and know where to look. Once the thermal shows where the heat source is you still need to identify what it is, and we spend at least as long doing this as we ever did scanning the hill with binoculars.