I've never been the caretaker for someone who was literally immobile, but I dated someone who was 600+ pounds and whose mobility was limited. She could stand and walk short distances, but it was hard work, and she used a scooter to get around when she left the house.
>>29289 has it right. First off, there's the smell. I'm not into gross/slob stuff even as a fantasy, but I wasn't prepared for just how bad it was going to be. It wasn't always at its worst, but when it was, it was *ripe* and you could smell it from a couple feet away.
And point 3 is an issue even before true immobility. If her mobility is limited enough that it's difficult to leave the house and do stuff, that means her range of hobbies and life experiences is going to be pretty limited. Maybe you can luck out and find an immobile but brilliant nerd goddess who uses the Internet to become a knowledgeable conversationist about a wide range of intellectual and artistic subjects... but in my experience, with pretty much every 500+ SSBBW I ever dated, her main "hobby" was sitting on the couch and watching TV.
If she has trouble leaving the house, how will this integrate with the rest of your life? Are you okay with her not being able to spend time outside the home with you and your friend group? Will you get bored being cooped up in the house? How often will you go out solo to pursue your hobbies while she sits at home? Will *she* be okay with whatever you choose?
And yeah, I never got to the point of moving in with her and being a live-in caretaker, but I thought a lot about what this would look like in reality. I'm in decent physical shape, but it was disturbing to think about how if she had any kind of health crisis, or if she fell and couldn't get up, I literally wouldn't be able to get her up and take her to the hospital by myself, I'd have to call EMS and tell them to send a bunch of people.
There's a lot about immobility that I find hot in fantasy, including the romance and intimacy of being someone's caretaker, and the sheer amount of gluttony it takes to reach that size. But after my brushes with the reality I accepted that this was something better off left as a fantasy.