The only way any chassis or crush structure helps the driver in a crash is through deformation of the many crushable structures around the driver. Without those crushable structures the driver will be in a world of hurt whether the chassis is tubular or composite.
My opinion is that a proper tubular structure can be very good with adequate crushable structures around the cockpit.
One of the biggest advantage to a composite chassis is that they are much "cleaner" on the inside so the driver has less chance of puncture or tube impact loads on arms and legs etc.