The downside to doing it piecemeal by yourself is the variances in parts/measurements/tolerances for each will vary. With buying a kit you're not only paying just for the kit/install but you're also paying for the R&D that went into the kit to make sure that it all works together as advertised. If I could buy all the parts and do it myself for $7k, and the kit is $10k, that extra $3k is fine because it's buying peace of mind that you know someone else already did the hard work and is giving you a completed project you know works.
But if you're feeling froggy and have the know-how to do it, by all means no one will tell you not to try.
