Guinea pigs are easier then anything to raise to and care for. They do just fine for me outside 24/7 even in the winter. Pups don't survive pupping in the winter though and it's best to bring pregnant sows in from January to end February if breeding over the winter.
Normal litter size is 3, have raised that average in my herd to 4, often five. I've had as many as seven born and survive in a litter though the two runts took forever to grow to a sellable size. Often first time moms have a dead pup or three in a litter, bred back to back dead pups rarely occur (at least in my herd).
They hit the ground running and are SO easy to care for even if momma dies. Pups are sellable at three weeks and males need pulling out if you don't want them breeding momma (some mature sooner than that even). Some advocate learning pups in with momma longer but I've had pregnant sows so I always pull the boars as soon as they act remotely interested.. often at just over 2 weeks.
Mites can be a problem. But you can breed for resistance to them. (or at least aim to breed a pig that doesn't stress out as easily and therefore become as susceptible to them).
Never had breathing issues.
They tame easily enough if handled regularly.
Generally solid floors are recommended though I've run them on wide spaced wire floors without issues, regular 1/2 x 1 i would NOT recommend (broken leg bones).
VERY easy to colony raise. Gestation is so long you can breed back immediately at pupping without harmful effects on the sow. Having a boss sow makes the herd run smoother.. two boss sows well... not so easy.... though adding a dominant senior boar helps that out nicely.
Pet homes need educating on how to handle to prevent breaking their leg bones.
Pros to keeping em?
easy, can have fast weight gain, eat anything I toss their way, make good critter food, raptor rescue places will say WOOT WOOT if you donate some to them.. they apparently LOVE them for the larger birds, easy colony raising.. SO easy.
Cons to keeping 'em.
misery to skin, no money in them for pets (Unless have a good contract with pet store), they are noisy (unless you actively breed for quieter animals),
Like my pigs but have decided that four-five sows with one boar is enough though at the moment I have no senior boar.. I had people wanting them and hopes that pregnant sows will have a son (or two or three).