Op, as said I agree it is not great or sustainable, but i do think you are looking at it wrong. Sure they allow people to eat sugar, but eating muller lights or hifi bars may be forcing many people to eat a lot less sugar than they were before the diet whilst still allowing them something.
There are many different diet plans, and many different ways of eating and it's important to find something that works for you. If something doesn't work for you then you find something else. Chosing to stay on it, making the wrong choices then blaming the diet is removing your personal responsibility. You chose to do that. So slimming world didn't leave you fat tired and addicted to sugar, you did that.
We all need to accept personal responsibility for our choices. I joined a slimming club years ago, I can't remember if it was SW or WW. but I knew within a couple of weeks it wasn't for me. It all seemed to be a bit complicated which led to an obsessive focus on food, the meetings were dull with the leader talking to the class like they were idiots about food choices, and the weigh ins and fake congrats and comaraderie were just completely cringe making, so I didn't go back and did it my way,
You can't keep on it, decide not to exercise because someone says you don't "need" to, eat as much crap as you're allowed then whinge and say it's all their fault. It's not, it's yours, I'm sorry.