Why not just use Full Flow Staged?

the full flow staged has a longer time delay between input and full thrust (spool up), and the engines are heavier - the instant and lower thrust of a electric power cycle on a docking craft or lander can be easier to control - but for takeoff and high twr burns, the full flow staged works great

if you want a real-life answer, the wikipedia states that "Potential disadvantages of the full-flow staged combustion cycle include increased engineering complexity of two preburners, relative to a single-shaft staged combustion cycle, as well as an increased parts count."

but i'm not sure why this would be a problem in simplerockets 2, i haven't even observed what type of combustion the community uses