Displacement works in the normal direction (perpendicular to the surface).  If two surfaces meet at, say, 90 degrees, and displacement is carried to the edges of the surfaces, they will appear to separate.  I don't know any way around this.  It is best to keep the displacement to details, and avoid going to the edge of a surface as much as possible where this will show.

There is a technique, using a color math node, that allows both positive and negative displacement on the same material.  This may be helpful in some circumstances.  If you're interested, let me know and I will search for ther thread.

John