I won't get into all the genetics of Lethals, but I do have one, so this is from experience. My lethal is Blind, Deaf, has teeth that constantly need to be cut down every few weeks and gets sick quite often. He was an accident from when my Pixie got pregnant becuase I thought my other pig was a girl too, and everyday I look at him and feel grief that I wasn't smart enough to properly sex my pigs. They require a lifetime of dedication, something I know some people won't give them. Some lethals are not even lucky enough to make it through birth and die right away. Ones the do, often don't live as long as normal pigs and die from complications that are often internal. I live in fear everyday that Edward (my lethal) will pass away. I'm trying very hard to make sure he's living as comfortable as I can make him.
When you breed, you're putting pigs in danger, both the momma and the babies. You can say all you want that you'll try and make that not happen or save them, but you can't change nature or play "god". And while that pig is giving birth, I can assure you, hundreds, maybe even thousands, of other are being put down or still waiting to go to good homes. Can you live with the guilt if the mother of the babies die? Or maybe even the guilt of bringing a lethal to life, who suffers because of it's complications?