I’m not familiar with Kiwi, but I noticed a comment thread about them on travel.stackexchange.com recently, discussing whether to describe them as “fraud” or just “using booking practices that subject the client to extreme risks”. Be sure to read the fine print on that booking!
The only problem with them is that they will search for and book flights with exactly the maximum luggage allowance that you specify, and they don't make this clear or really point out that you need to specify an amount of luggage corresponding to the amount you'll be taking with you.
They will also show you itineraries that result from booking separate legs of the flight separately, which I'm guessing is what you're referring to. That's not in the fine print; you cannot possibly avoid being made aware when that's happening.
