Extreme obesity

A real FA would be at hand to carry his SSBBW around the house when she needs it.

Don't listen to her, she's just here up be a killjoy. Immobility is hot, but it tends to be relatively short lived because it requires the most care and causes the most health problems, so at that point you either fatten until you die in a few years, a great way to go, or pragmatism/finances/fear take over and you lose enough to somewhat take care of yourself. This is why riding the line of immobility is also pretty hot. By the time you're big enough to almost be immobile, for it to be a real worry, you're only one small injury away from that tilling point. Hurt your legs, hips, or back in any way and by the time you're healed you'll be too weak and fat to get up without help. Even if you're vigilant and try your best to diet and exercise, a sprain could keep you in bed for a year. At that point you might as well just accept your fate and let your evil feeder stretch your skin to it's limits.

That being said there are many super fats who have lived contently and fairly independently while riding that line. Summer was massive, over 650lbs but she did mostly everything for herself and was very happy slowly meandering about on her own and taking care of herself. Gina could get up (with grabholds) at over 700 lbs. Ash has been massive and maintaining for quite some time, and Heather is over 600 and in her 50's and she slowly gets around. Echo is barely mobile but she can still stand after enough rolling and shimmying. Vanillahippo is 800 lbs and can still stand without help. If you can't stand without help, and you have a feeder, you're passed the point of no return even if you can still walk. You'll find them helping you less and less and you won't complain, you'll grow complacent, and get off on it, and tell yourself you'll do something about it later...but you won't

You can't predict. My wife is not a feedee. She is average height and has never been close to 300 pounds. Recently, her knees and hip has given out and she is very immobile. Age has a lot to do with things. You may not have a problem being young and very fat, but add age to the equation, and you may become immobile.

All things being equal, studies show if you are obese, it's better to be pear shaped than to carry a lot of visceral fat.

Cartilage loss in the knees goes up even when you are overweight, and is near exponentially worse as you gain over 300 or more pounds.

Of course there are also the problems of cardio vascular issues, type 2 diabetes, certain cancers, higher risk of dementia in old age, hormonal problems and so on.

I am not trying to discourage you at all, just since you asked about the risks. I don't think anyone intentionally becomes obese for health benefits. It's like ultra running, no one goes out and runs 100 mile plus races because its good for them, they do it because they want to.