I use arcpy_metadata to bulk update ISO 19139 format metadata for ArcGIS 10.4-10.7
It works well for the basic fields that you may need in a company. You cannot update every metadata entry with it currently.
My files tend to follow a naming convention where underscores separate the name components e.g. topo_NSW_Lidar_2015_AreaA_05m_DEM.img so my code separates the name into keyword tags.
It's very easy to update just one or two of the fields where they are all the same info e.g. the place keywords or the processing keywords.
I generate a file of Descriptions and supplemental information in Excel and read through that to update individual files where they need something different from the en masse processing
It's unbelievable that ESRI have not built in the ability to work with metadata in Python as metadata is the greatest chore in GIS management.
I personally wouldn't waste my time on etree as arcpy_metadata has already dealt with most of the issues.
Neil