XRAY - Model racing cars

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• Longer top deck design minimizes tweak and makes the car more stable ....
• New rear split bulkheads allow more rear flex, and improved rear traction ....
• XRAY Multi-Flex Technology™ (XRAY patent) allows for fully adjustable chassis and top deck flex and torsion .....
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I'm a little confused about these, for me conflicting, quotes.
In my undesrtanding a good, easy to set up, car has a low CG, all options
to set up suspension geometry and characteristics, low weight of unsprung
masses - and a chassis as stiff as possible.
Seperating a stable body that holds the car together, from the dynamic
system of suspension.

Can somebody explain why a flex in chassis is ever wanted and why these
effects can't be done, in a more predictable manner, by suspension only

You are correct that the hardness of suspension plays important role too. But you can be sure that you can not go with extremes on both sides (chassis flex and suspension flex), which means you can not compensate one with others but you need to find a best compromise between both chassis and suspension flex. Therefore for foam racing on carpet, which is done typically on very high grip racing conditions, the hard chassis (minimum flex) and hard suspensions work best while for rubber tires racing you need to find the correct compromise of the chassis flex and you need to go with a bit softer suspension.
But remember that we have been talking about high competition racing where you have been looking for the ultimate small performance difference. For the typical club level racing if you will go with the basic suggested settings you will have a good handling car and you can experience from there.