I concur with what you say about feral cats' systems being compromised, Isi. When I went out to Saudi, there were many ferals (and I guess there still are) roaming around. The oil company used to have cull (humanely trapping them or darting those who wouldn't fall for the trick), putting them all to sleep. The first cat I had was a female from a very wild, all-black female who wouldn't approach people herself, but who very cleverly insinuated her baby into our shared dormitory house.
I called her Sammy and I soon found out that she was already pregnant. She produced three all-black female kits and one all-white male, who was stillborn. I found home for two of the girls, but the third was very tiny and wobbly, and eventually after having her tested, the vet decided she was spastic. Her name was - er, Kitty. Sammy lived to be 12 before having to be pts after a very rapid weight loss - cancer had just ripped through her. Kitty, tiny, wobbly, and very peevish to the end of her days, lived to be a good 14, before her system just seemed to crash very quickly, and just as the vet was trying to rehydrate her, she passed away.
I took on Sunny, a very feral Mum and her four kits. Sunny lived to be approximately 10 before succumbing to kidney failure and one of her female kits, Poppy, had to be pts because she contracted FHIV. The little boy I kept, Rusty, suffered from a thyroid condition which stunted his growth and made his fur very coarse. They all developed wonderful, individual characters - Rusty was tragically killed when following his sister across a road, when she returned to the house they'd all been born at and lived in. I had to move when the company decided to demolish the houses, and I blame them for Rusty's death!
But, as you can see, while most of them lived fairly longish lives, they all suffered complications with organs or growth, due no doubt to the rubbish fare their poor mothers had to forage for when pregnant. I feel they enriched my life and have left me wonderful memories of fun with them, though, and I hope I gave them all longer, happier lives than they'd have had otherwise.