Really though it's a tongue-in-cheek observation with a basis in the reality that a 'Joint' program implies multiple service branches sourcing requirements. In turn, that increases requirements count and complexity, which in turn has a non-linear and negative effect of program efficiency.
It also often moves program decisions from technical tradeoffs made by engineers into political space between branches.
https://en.wikipedia.org/wiki/AGM-169_Joint_Common_Missile - cancelled
https://en.wikipedia.org/wiki/Joint_Air-to-Ground_Missile - follow on to the JCM (not sure about it's status - they may have gotten the planning right by putting in for a long dev period up front)
Really though it's a tongue-in-cheek observation with a basis in the reality that a 'Joint' program implies multiple service branches sourcing requirements. In turn, that increases requirements count and complexity, which in turn has a non-linear and negative effect of program efficiency.
It also often moves program decisions from technical tradeoffs made by engineers into political space between branches.