So from what I can tell, the ball physics are fantastic. The flipper physics however could use some tweaking I think. I immediately noticed that the flippers seemed a bit too powerful in many cases. For example, post transfers seem to send the ball flying a lot. After playing a bit more, I think I realized what exactly the problem is. In real pinball for example, if you hold the flipper up to try and catch the ball, but the ball is going a bit too fast and will roll over the edge, you can quickly let go of the flipper and then press it again to give the ball a small tap. This usually gives the ball a little momentum up and back towards the flipper's respective side of the game, allowing you to safely cradle the ball. In Zen however, I noticed that whenever I try to do this, the ball will go flying.
What I think is happening is that the Zen flippers are hitting the ball with pretty much full power whenever they're moving, regardless of the length of the stroke, when the way it should be is that a small stroke should have a fraction of the power of a full stroke. So for example, if the flipper is starting from 0% of it's swing, it should become more powerful as it progresses through that swing, and since the stroke is so long will have plenty of power relatively early in the swing. If the flipper is starting at 90% of its swing however, when it is energized it should certainly not have the same strength as a swing starting from 0% would have at 90%, rather it should have roughly the same power as the 0% swing had at it's start.
Now I'm certainly not a physics expert, but this has always been my understanding of how flipper physics work. I think that if more realistic flippers could be introduced into the Williams game, it would really kick the whole experience up another notch!