Have any of your restored a antique cast iron farmhouse kitchen sink? Is it a difficult job?

If you’re talking about the enameled, cast-iron type then yes, I think it would be difficult.

I don't know how to restore one. I used to have one that was also the counter top in one piece. Single deep sink. I bought a big modern two sided cast enamel sink when I re did the kitchen as on other house years later. It looked nice but after using it I prefer a deep stainless. Dorp a glass or plate in cast and it will likely bust but it will usually bounce in a stainless sink. Also why I'm not a fan of stone counter tops or ceramic tile in the kitchen.

They look amazing but have some disadvantages.

We have done a few clawfoot tubs, no sinks though. You'd be surprised what you can get done with Comet and a wet rag. If the enamel is chipped it's stilll repairable but a little tougher to do. It can be a diy project though with a little research.