Hm, that's not really that good.
There are many, many things with metadata processing which can cause issues.
But hierarchical keywords is a quite simple tag, basically a bag of strings using | to indicate hierarchy levels.
I wonder what else they do.
Can you send me an original and an image processed with DxO and Nik Collection so I can do a comparison? You can send the files to
support email address (please also include a link back to this thread, because I get many emails per day).
The problem is that XMP has two sets of keyword (flat) and hierarchical. Which must be synchronized somehow.
And if the file also has legacy IPTC metadata (and many older images have), you have to deal with a 3rd set of keywords (which even has length limits).
IMatch internally uses hierarchical keywords, because the the most flexible variant.
And when it
flattens the keywords in the "XMP subject", it offers several ways to do this (See Edit > Preferences > Metadata).
These options allow users to best match the requirements of their other applications.
When
importing metadata from images, IMatch uses it's
universal thesaurus and some smart algorithms to cross-reference and import hierarchical and flat keywords. To avoid producing duplicates or accidentally creating additional top-level keywords from flat hierarchical keywords. This is tricky and requires care...probably here your other software has some work to do.
Dealing with this (and the many, many other metadata pitfalls) is hard. Really. And requires constant changes and additions.
it is also totally unsexy.
It is never tested in magazine reviews or examined in the cool YouTube hype videos.
It cannot be shown in ads or comparison tables with other products - which is what product marketing cares about

Hence, even prominent products skimp in this area or even totally disrespect standards, to make things easier for them. Or to look customers in to their product or service.
Users often only notice this when they look at their files in other software. And then find metadata missing, wrong or damaged.
Since IMatch is using the awesome ExifTool and actually cares for all this metadata stuff, I recommend only using IMatch or an equally complete and capable software to modify the metadata in your images.
Your advise is sound and I fully agree.