The reason why it's terrible is because mental models are like a sort of jumbled set of tools, copied over from somewhere else. Deep and genuine knowledge is tacit, integrated, incorporates experience and is organic. It's acquired through training and expertise and often can't be verbalised or formalised.
Relying on mental models is a sort of crutch. Maybe a good comparison is chess playing. If you're a new chess player you learn that pieces are worth X points, and that such and such squares are important, and that's basically like these sorts of models. Really good chess players don't consciously think like that, at least not in a sort of pronounced way. They've incorporated knowledge and play by intuition. They can make 'good moves' in a second or two.
I don't know why these 'fallacies' and 'mental model' things have taken off so much within certain circles, I feel like it's because people do nothing else other than reading these blogs. Real expertise or knowledge isn't gained by sitting around and 'thinking through stuff', it's by coming into interaction with the real world and immersing yourself in what you want to get better at.