As a 32 year practicing periodontist in the commenwealth, I would deem it reckless if any dental assistant performs scalings (supragingival or subgingival) on patients.  If can allow ineffective cleaning of the teeth, it can also allow rough tooth root surfaces.  While it has been difficult to find a hygienist in the past, there are now ample hygienists to provide services under existing regulations.  Dental assistants would need to have the same training as hygienists and even many hygienists have not received quality training in scaling.