The system's stiffness is but one aspect that affects the feel and function. Some of the stiffest brakes, WRT the lever pull, I've felt are cheap stamped steel units found on many English 3 speeds. But their leverage leaves much to be desired. Then add in the hard last a lifetime brake blocks (not sure "pads" is the right term ) and slick in the rain steel rims...

Back to center pulls- Yes there is a greater chance of more flex in the system, The hanger can bend under cable pressure, the straddle cable gets drawn taught, the arm pivots cab be sloppy. But the leverage factor is often greater too. As I alluded to before a well sorted out center pull system will have great leverage, be stiff enough to not bottom out a lever, have a lot of tire/fender clearance and it's adjustment state will last a long time. But some of the same can be claimed for a side pull system (the bike I'm about to ride has 30+ year old campy NR side pulls that will go for another 30 years). So you picks your poison. Andy.