The FreeCAD material editor has improved see git commit 195c530 and topic https://forum.freecadweb.org/viewtopic.php?f=38&t=28952
It now supports units. On the long run editing materials should only be done in material editor. It is possible to start the material editor from FEM material task panel already. Would be cool if some of you could start to use the material editor, like I did some time ago and report any issues to the topic mentioned above.