Cons: Simply the two biggest are... You are giving your sow about a 20% of dying from birthing complications. You're adding to the overpopulation problem. There is no purpose to just breed cavies just to sell them or just to have pups, or just for the heck of it. You can foster a pregnant sow from the shelter or pound, and then you can take care of pups, without breeding them yourself just for pups, or another reason other than improving the breed.
Cavies are a lot different from a rabbit; they are born fully furred, eyes open, and reading to run about and squeal. They need to nurse for 3-4 weeks. Some will nurse longer. Females can be with their mom, but males must be seperated at 3 weeks, because they can become sexually mature at that young, and can even breed with their mom, or possibly sisters.
Over 8 months, and the chances of the sow dying from birthing complications REALLY ups! It's risky to breed sows over 12 (1 year) months of age. The best age to breed them for the first time is 4-6 months.
Cavy sows do not need nestboxes. They just have the pups in a corner or in a house they like. Babies are born fully and like marathon explorers.
They aren't little things when born! They are whale size; 3-4 ounces on average! A pregnant sow carriers 10% of her weight in pups!
Sows have heat cycles, unlike rabbits, which can breed anytime in the right conditions. They go into heat around every 16-21 days for a few hours, like 10 or so. You'll know when a sow is in heat if they live with company... you cannot miss a sow acting like a total boar! If a sow is rumble strutting, purring, mounting other sows, then she is either in heat most likely, or showing dominance.
It's not too easy to find homes for new cavies. There are cavies in shelters, the "Opps, my little boar got into my girl's cage!" litters. Which mindfully are trying to rehome their cavies too.
Prepare for miscarriages, birthing difficulties, pregnancy toxemia, etc, because they all can happen... usually when you least expect it.
Breeding before you know proper cavy care isn't responsible. You should at least attend some shows before you start to breed. Learn how to properly care for cavies, learn how fun and interesting they are, learn how to just enjoy them as pets, just have fun with them before you decide to breed them.
Sure they may be good Silkies, but unless you are really going to get into cavies, what's the purpose to just add to the overpopulation problem?
A lot of the cons of breeding animals is emotional stuff. It's emotional when animal die from birthing complications, or when stillborns are born, animals have miscarriages, etc. Tyhe other part is for the animal's sake. I have NEVER had unpregnant sows ever want to be pregnant. Cavies would much rather be snuggled and loved than go through pregnancy and give birth to those giants.
Pros: Babies are awesome... Showing is fun... and hey... you can meet new people, and have fun.
Before you start breeding, my opinion would be for you to really get into cavies just as pets for now, and then after a little while, attend some cavy shows, talk to other breeders, and really get what breeding and showing is all about, and then maybe breed cavies.
Good luck to whatever you decide to do, but please just remember that then only reason to breed is to improve the breed, since there are many cavies in shelters and pounds that need homes too.