As I recall from having brushed up against UniData a few years ago, this class of database system works well for limited-depth hierarchical data because of the various things you've mentioned, but that doesn't necessarily mean that all of the advantages apply to other kinds of database structure.
I think it's also pertinent to mention that PostgreSQL has been able to deal with things like multivalued columns for a long time and in an arguably more sane fashion than, say, UniData in various respects, such as in the storage representation which, as I recall with UniData, involved various "top-bit-set" characters as field boundaries that probably made internationalisation a pain.
Certainly, this class of system works well for certain kinds of systems and there's undoubtedly a lot of people still sticking with them, as well as a lot who tried to migrate from them in a bad way, either causing lots of teething troubles and organisational stress with the new system or reinforcing prejudices about the capabilities of "those new-fangled client-server RDBMSs". That the migration exercises probably involved Oracle or some other proprietary solution, where only a single vendor can ease the pain, probably won't have helped.
It's telling that UniData and UniVerse ended up ultimately with IBM after an acquisitions cascade that involved lots of slightly bigger fish eating other fish. I think it was Ardent acquiring the Uni-products, being acquired by Informix, being acquired by IBM. Unlike HP who would have killed the product line in order to add a few cents to the dividend for that quarter, IBM probably see the long-term value in those remaining customers.