I'm so sad to have to open this ticket. I found another case when the boolean modeller creates an invalid mesh.
See the scene attached: I made an intersection of two extruded objects, then converted the result to a triangle mesh. The solid editor shows an impressive number of irregular edges and faces.
BTW it is also weird that a 10161-faces mesh results from the intersection of two 60-faces ones!
The work around is the usual one: move one of the object by a small amount (e.g. 0.001) on the x axis and all works fine. Therefore the problem seems to be limited to the case of coplanar, overlapping faces between two objects.
I used to fix myself this kind of bugs in the CSG modeller so far but I can't afford that any more, not in the near future. Since a work around exist a fix is not urgent to me, but I wanted to share the issue in case someone volunteer to work on it.