I think the "one golden shot" as it's sometimes called, applies to all diets and I think a lot of it is around the commitment. In low-carbing you succumb to carb-creep, if calorie- or point- counting you risk getting blase about weighing and measuring.
I did the Carol Vorderman detox diet a few years ago and lost 18lbs in 21 days. I've never been able to stick to it since for more than a couple of days at a time. Silly because it works for me.
Add to that your body's intention to hang on to your fat and place it round your middle as you get older...
It's a bit like keeping a relationship exciting when you've been together for a while and not taking each other for granted maybe. Try new things, new foods etc within the diet you're following so as not to get into a rut.
Well, that's the theory sorted - better get onto the practice then!