I have just recently returned from a trip to Europe, and although I was originally very much happy with the price of my itinerary - I have occurred multiple baggage fees (adding up to additional ~ $300) which spoiled my overall satisfaction with Kiwi.
I do understand that our tickets were booked in economy class (no surprise there) & I did check in very detail w/ every airline we were booked on in regards of specific luggage allowances & possible fees. However - accessing the Kiwi-purchased reservation on the airline's website, the customer does not learn about the 'underclass' (= lite) in which the economy ticket was purchased & additional luggage fees. If no advanced luggage fee is purchased, charges double (or triple) if required to buy at the gate, while boarding. On the other hand -- while returning back to the US, and after 'lesson learned' from the outbound part of our journey -- I have purchased additional luggage services ahead of time for our return flights (following the airlines' website guidance) -- to our surprise -- this was not required, and I waisted money on these additional fees.
So -- I understand that each airline has a different luggage allowance, however for Kiwi customer, it is pretty much impossible to determine what is allowed and what is not. The economy-lite booking underclass allowing passengers to travel sometimes w/ personal item only, sometimes w/ personal item + 1 carry on piece is very much undiscoverable until you get to the gate (= too late).
So be aware: saving $200 on airfare might end up causing much more on extensive luggage fees.