Long term these shakes/bars don't work
Bit of a nonsense phrase that isn't it? It doesn't actually mean anything
If you stuck to the shakes long term you would continue to lose weight until you're weight reaches a level that it will maintain on about a 1200 cal diet. that sense if you stick to the shake diet, it will work. The shakes won't suddenly stop working.
If you return to your normal eating patterns when you "finish" the diet. You will put on weight. That is the same with any "diet". If you go back to overeating you will go back to putting on weight. That's not the shakes not working that's your choice over what you put in your mouth.
Most people won't sustain a shake diet long term, at 6 weeks. And the problem with the shakes is that they don't teach you what you should be eating in the long run. That doesn't mean the shakes don't work, they do, it just means you need a sustainable long term plan that suits you. They also don't provide enough calories if you do anything more than the most limited exercise or a highly active job. These are all valid critisms of slimfast
And for all the fuss about sugar, when i was tracking my nutrients I did find slimfast one of the easiest most effective ways for getting all the nutrients you need on a 1200cal diet. It isn't easy otherwise. Your shake might be less calories but that's only good or bad in the context of the rest of your diet either in context of nutrition or calorie values.
Long term no diet, either maintenance or weight-loss, will "work" if you don't keep to it. People do shakes because they they are a simple way to lose a lot of weight quickly. Long term weight loss/management takes dedication and commitment, but that's the same whatever method you use to manage your food input.