In this case you need to graft the planes and not the pipes.
Grafting the pipes leaves you with a data structure per pipe.
You want to have sections per plane.
In your existing definition you can look at the Curves input of the Region Union component and you’ll see paths with different lengths. Due to that, the Region Union component doesn’t know what to do.
I added a Shift Paths component to have both the pipe sections and the metaball curves with an identical data structure.
You will also have to move one point. Easiest way is with the preview of the point component on. Pull an arrow of the point gumball. It does not accept numerical input unfortunately.
METABALL EXPLORATION - ROUGHy.gh (32.8 KB)