Omerta:
I hate this question, because the best answer is "it depends". It depends on which kit, which juice, and which old winemaker.
Apparently he likes terrible wine. Is that because you don't like what he makes or because it is badly oxidized (or some other fault)?
It is really hard for me to tell.
I can give you my personal opinion based on making MANY MANY kits, and ONE crappy pail of juice. Can you guess my opinion yet?
Personally, I think that a good quality kit, made well, will be better then your father-in-law's wine from juice. But it may not impress someone who's taste buds are used to their own "terrible" wine.
But if he is using a decent quality juice, and following reasonable practices, it is probably a reasonable wine. And maybe it's your tatse buds that are at fault.
If he makes red zinfandel and you love zinfandel blush, then never the twain shall meet, or at least not until yours grow up.
Steve