While other VoWiFi phones only aim at professional environments, like the Unify/Ascom series, the Grandstream phones are more affordable and its functions closer to private customers.
I personally have a Grandstream WP810 because of the smaller form factor compared to WP820/822/825.
The feature, missing the most, is a way to sync the phonebook with my other DECT phones, the family uses. CardDAV is supported by them and would be a great way to exchange the data with the WP810, too.
While LDAP is common in companies, CardDAV can be used by private users easily. Thus, I’d love to see it in one of your future firmware updates.
I managed to convert CardDAV to the WP810’s format but there is not automatic updating which makes it difficult.