Warning: Do not try this without saving first.
The attached file has one polysrf. If I run MergeAllFaces on this polysrf, Rhino V6 Mac hangs.
Problem Merge.3dm (3.3 MB)
You can’t merge trimmed surfaces. Trimming only really hides a part of a surface from you, its still there. Use the shape you have to create curves and recreate surfaces patches. The large tapered face will create three patches but you can merge them to one, sometimes its not a good idea to do that though, as you can end up with creased edges, but you can resolve this by running DivideAlongCreases.
Problem Merge_Fix.3dm (299.0 KB)
But does it cause a hang for you as well?
No it didn’t hang. I just get the message “Merged 0 polysurfaces. 3 polysurfaces could not be merged further” (because you cant merge trimmed surfaces)
I am on a windows machine. Perhaps its an issue on mac, give it an impossible command it gets grumpy?
You can certainly merge trimmed polysurface faces with MergeAllFaces - but only planar, coplanar ones. That’s why it has been renamed MergeCoplanarFaces for V7.
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I stand corrected 
I am curious whether this hangs rhino for others. I’d like to know if this is a system problem on my end.
The work around for this easy: remove the faces and cap.
The problem with this file is the seam of the circular surface on the top. It lies exactly on the point of the trim edge of the surface adjacent to it.
run srfseam and rotate the seam 90 degrees from where it is and the command completes as expected.
(tested on both mac and pc v6 and v7beta)
Hi -
Yes, this caused Rhino 6 and 7 to hang on both Windows and Mac.
RH-60954 was fixed and tested and will be in the next public Rhino 7 beta.
-wim
This V6/V7 script can merge the faces of your model: