Here's my thoughts on chassis flex. We have driven the state of development of R/C vehicle chassis to an extremely refined level. The tuning options and resolution of them is incredible.

Stiffer was better for a long time in following with standard race car practice from full size thinking. This was especially true during the foam tire touring car era. Grip available was so high that a flexy car was pure evil if not set correctly, or at the very least off pace.

Now that rubber tire is the standard we have to factor in tire loading and heat along with setting the car for driver and circuit. Load the tires too hard, traction goes away. Not enough, car is off pace or breaks traction easily. Traction level going up over the course of an event (primarily carpet but asphalt builds in relation to weather) means you are always chasing the correct setting for optimal tire loading.

With the highly refined level of tuning now possible from most chassis, finding the correct setting can be difficult. Getting right up to that 'magic' point where the car is truly neutral the entire run is very difficult. With a stiffer chassis, driver input is also much more direct and if you're off enough on settings, a car that is good early in the run may 'go away' by the end of the run.

One other factor that complicates what we do with model racing cars is feedback. Quite simply, we do not sit in our cars. We do not even 'feel' what the car is doing. Drivers tend to use that term to describe how the car reacts to input on track but there is no physical feedback from the chassis. Without that feedback we do not know when a car is near the limit of traction. We simply react to it after our eyes see it and it's processed by our brain and hands.

Long story short, a more flexible chassis with finer tuning results in better performance. This is because it is easier to drive near the limit of traction without going too far and overloading the tire. Think of the flexible chassis as a two way filter, both for input from the driver and the racing surface.