The disadvantage with "pre made" anything is that they end up being either prohibitively expensive or functionally deficient. It's understandable, people and businesses aren't expected to work for free, and even if they did work for free, they have overhead costs to cover. Look at first aid kits, for an easy low hanging fruit example. Would it kill them to use high quality flexible fabric bandaids that will actually stick to your skin? They use the cheapest generic plastic, guaranteed to fall off within hours useless pieces of crap. But in order to bring an affordable offering to market, they have to fill it with the lowest cost/quality crap on the market. On the flip side, if you get a first aid kit filled with high quality stuff, you end up paying hundreds and hundreds of dollars for a lot with components you could buy at Walgreen's for 1/5 the price.
The attraction of the pre made kit is that it takes the work out of it. Someone else with knowledge and experience has put in the time and effort to determine what a person might need. If I was making a first aid kit, I'd probably end up putting bandaids and superglue in it. Someone with more knowledge and experience would have a much more comprehensive packing list, and about the time I was sitting on the ground with a broken collarbone, I'd be tickled pink to discover that there was some rolled up fabric for making an arm sling.