Kiwi.com for Japan travel specifically — is it worth using for domestic Japanese routes or just for international flights

Been planning a Japan trip and someone suggested checking Kiwi.com for the international leg but I'm also trying to figure out whether it's worth using for domestic flights within Japan or whether booking directly with ANA or JAL is always going to be cheaper. Has anyone used Kiwi.com for Japan domestic routes recently and found it competitive or is it really only useful for international and cross-border bookings.