Kits are guaranteed while juices arent because all the balancing is done with the kits. With juices you need to sometimes adjust TA and PH which can mean doing malolactic fermentation. That is a second fermentation where you add a bacteria in to your wine to eat up the malic acid and turn it into lactic acid which is much smoother. Im not to fond of the juices much myself, Ive done a few with not so good results, didnt think the juice was that good to start with and the end result wasnt great either. I tried 2 brands, one was a little better then the other. Ive made wine from kits juices and grapes and the kits and grapes IMO make better wine.