All of my cats are indoors all the time. They hate it outside anyway, even the formerly feral kitty, as i try to get them to sit outside with me on nice days and run around (on a harness and lead, of course). They will usually have nothing to do with it. I have heard people say that they have a cat (often semi-feral or formerly feral cats) who would be absolutely miserable indoors, which is a bit of a different situation, I suppose.
The rescue group i work with does a lot of TNR and we also take feral cats that the local shelters pick up and take them in so they are not automatically euthanized. Plenty of the cats that we are called to "trap" turn out to be people's pets-- it is really sad. We also have gotten a number of calls to trap in areas where outdoor cats are being poisoned. There are just too many things I would worry about. Maybe if i lived in a nice, out-in-the-middle-of-nowhere kind of place I would feel differently, though.
The absolute worst is when they are declawed. No cat that is declawed should EVER be outside unless their owner is out there with them giving them constant supervision. That is just cruel and ignorant, in my opinion. I used to work at a vet clinic and thinking of some of the things i saw happen to declawed cats that were outdoors still makes me shudder. Of course, terrible things can and do happen to any outdoor cats, but when it happens to a cat that a human rendered hepless and defenseless, that just makes me sick.