I have tried KJF with some succes. How I play it is that I never decide to go KJF until J1 has passed. That means I will usually not devote any Russian or British forces to fight Japan in the first round. Instead I will focus on the European theater and Africa first, building a UK navy and trying to sink the Med Fleet ASAP. The reasoning is that even with a KJF, you can’t let Germany just run rampant. KJF often means a long game, so keeping the African IPCs out of German hands is very important. With UK that will always be my number one priority, so it’s safe to say that I will never build an Indian IC on UK1. Of course, the Japanese transport will still be sunk in 99% of the games.
For me, the most important KJF triggers happen on J1 and can be summarized as Japan failing to take a critical objective (China, Pearl) or losing a lot of air and naval forces across multiple battles. If Japan does ‘big Pearl’ and takes heavy losses, exposing a battleship and carrier to a promising US counterattack, I will often go KJF. Same goes for ‘small Pearl’ and losing that battle. For losing China it’s a bit less clear-cut, since Allies often don’t have the units in that area to press on an advantage. In other occasions Japan could lose a BB in SZ59, or fighters in any battle, giving enough incentive for US to build a Pacific fleet.
After round 1, even if US went Pacific, I will mostly use UK to fight Germany and assist in the defense of Russia.
The sad thing about this form of KJF is that it is contingent on bad dice for Axis, and not used as a general strategy. The question is whether in those scenarios where Japan has bad dice, KJF might be viable but KGF could still be strictly better.