Human-animal barriers are hard to pass for infections (bacteria or virus) because there’s very little material shared between organisms: occasional spittle of saliva, maybe blood drops during butchering, but not much. Therefore, there’s hardly ever an opportunity to mutate in an environment that would select for human compatibility. In a chimera, there’s a life-long tissue-sized opportunity. Even for one individual, hat’s enormous.
If pigs routinely carry a pathogen to which they are immune or that has no apparent symptoms, we wouldn’t know. We know it happens, but we have no more way of knowing than Christopher Columbus did. If any pathogen proves easily transmittable but deadly to humans, it’s over. Every bridging case tells us such infections are likely, numerous even.
Chimeras are the perfect ground to mutate those until it’s too late — not for the individual recipient, but potentially for the entire human population. This is a very easy-to-imagine and, overall, quite likely extinction event.
We don’t know what pigs have that could be it. We could and do raise chimeras in total isolation, hoping they would not catch anything like that. Still, every step of the process, from the circumstances of the pigs’ birth to where the food comes from, is an opportunity for such contamination.
Philosophically, we know, but we don‘t know: we know what it would look like, so if nasty things happen, we would understand; however, we have no sure way to prevent it until it happens. The closest equivalent in fiction would be the opening chapter of _The Last Of Us_ (spoilers): tell every epidemiologist this is it, set yourself on fire, and beg a foreign country to immediately nuke your country out of existence, hoping that will be enough.
I find that new conception of risk interesting, but have been pondering that scenario for decades (one of my professors was the first to suggest that path). I realize that it sounds a lot grimmer than vegans and Muslim people finding the practice “revolting” without nearly as dark a shadow over the practice.