I use the Furniture Library Editor for dozens of custom models I have created, divided in multiple libraries, and I have never had any problems with it. Sure, I have some wishes to make it even better like selecting models and move them to a new or other library but I wouldn't want to edit the library in Sweet Home 3D itself. You have to understand that if you update a library it replaces everything so any edits done in Sweet Home 3D would be lost. It's better to keep the Library Editor as a separate tool.