I think its a combination of what you make of it, with a side helping of it might just not be the diet for you.
Slimming world does have a problem in that it is very possible to overeat and to play the system once you know what you are doing. So you can be 'on plan' and still eating more than you need. They call all these foods 'free' but they do have calories, espcially the pasta and potatoes, so you do need to apply some portion control.
However, I also think it gets some things right. It goes heavy on the fruit and veg (1/3 plate speed food) and tries to give you some outlet for cravings without totally free reign, and I do find going to group a good accountability tool. Calling excercise 'body magic' is exceeding annoying, but it is addressed.
I try and use most of my syns in making nice meals (not dry 0% fat, quark fulled affairs) rather than snacks, up the fruit and veg, steer clear of the Hifi bars and it is working again so far (joined again post baby).
I tried second nature and the online aspect just didn't work for me.The meals were really good but I found the low carb, high fat diet they push just unsustainable. I would feel totally wiped out mid afternoon, and I don't get that with SW, but perhaps that plan might work for you?