First of all,

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Yes, it should be safe because you choose the food yourself as long it's within your points. There are no specific foods or anything you have to eat. They do have healthy guidelines for you to follow like healthy fat (olive oil, nuts, avocado, etc), water, fruits, veggies, whole grains and such. I personally love WW because I think it's sustainable like something I can do for the rest of my life and I don't feel guilty if I want to indulge in one meal or one day, etc.
As for the diabetes part, I don't know but I would assume by eating healthier and making better choices, it should be manageable.