The US Congress Research Service has produced a report on the F35/JSF programme (https://crsreports.congress.gov/product/pdf/RL/RL30563).

From a support/development cost and interoperability perspective the paper describes one of the original objectives of the JSF programme: between 70% and 90% design commonality across the three aircraft variants: US Navy, USAF and US Marine Corps. However, what has actually been achieved is between 20% to 25% design commonality.

I believe that this lower level of design commonality in this programme can be explained by the phenomena of design divergence (Cardiff University research - Dr Ashley Morris PhD).  Design divergence is a mathematical property of all complex products that drives increased support costs.

I'd be interested in any observations of engineering behaviours that you have observed that might also be attributed to the phenomena of design divergence.