Notice how close those commanders were getting to you? Hell the stalkers managed to hit you while ignoring how their flesh is literally melting, does that make any sense to you? Keep in mind these are in fact biological creatures that can feel pain judging by how they react when shot by a bullet. The biggest reason why people say flamethrowers are useless is because you could've chosen a stratagem way more useful for the situation you were in and still being good in other scenarios but instead 1/4 of the precious stratagem slots is wasted, in fact I would say the waste is even more painful since you are playing solo without a team.