I've made a couple of wine kits, but I tend to stray from the directions. Most of the kits come with EC-1118 for the yeast because it is reliable, and ferments completely with little likelihood of a stuck fermentation especially for new winemakers. It can usually get the job done without a complex rehydration procedure, so they keep it simple and say "just sprinkle it on." They also are often designed to be done in 6 or 8 weeks and so they contain fining agents and other additives to speed the process along. Many will contain oak chips or dust to provide the oak.
Now me, I like to vary the yeast, so I'll choose the strain I want, then I will rehydrate it using GoFerm so I get a healthy start and complete fermentation. Then I let it sit. As a meadmaker, I'm in no hurry to bottle anything, so I tend to leave out the fining agents and just let time work its magic. I also prefer to use oak cubes or spirals. If my batch finishes dry, I will usually leave out the sorbate - the manufacturers don't want inexperienced winemakers to get bottle bombs with an incomplete fermentation - I am comfortable with preventing this.
So far, my deviations have not resulted in any stuck fermentations or spoiled batches so I suppose all is well. The kit manufacturers have to make a product that a beginner can use successfully with minimal risk of problems/complications. I do think, however, as meadmakers we are used to dealing with difficult fermentations and long aging so I don't feel nervous in abandoning the directions.
Kits are fun, and can make some very good wine, but there is something about starting with grapes that seems like it is missing in this - a bit like using malt extract with hops included in making a beer.
Medsen