Merge Faces bug?

Do you have the same problem when you want to merge several faces and loose continuity ? May be I don’t use this tool as it should be ?

That appears to be a SubD object.
What do you expect the MergeFaces command to do?

This is a SubD object and I want to simplify it before convert it into nurbs polysurface.
My goal is to have a model with less surfaces as possible.
It would be helpful to merge faces without having a crease in the middle. Thk you

If you delete an edge loop (Ctrl+Shift+Click or SelEdgeLoop > Delete) instead, you’ll maintain a quad structure while reducing faces. This will result in smoother topology. What is essentially happening in the case you showed is that an Ngon, if it were a mesh, is replacing several quads. Over large areas, faces with many edges will create puckers like this.

If you used QuadRemesh to create the SubD, you can reduce the face count there as well.