JSON-LD allows us to marshall queries into and out of the database as objects in their own right.

The verbosity isn't a problem as we don't actually write queries in JSON-LD. We use a fluent style in a programming language: currently, either Python or Javascript.

OWL as a schema language is quite rich and well developed allowing multiple-hierarchies and complex constraints. Having a well defined rich schema language for graphs is extremely important, and something that isn't widely available.

Are you looking at CQL at all? https://www.categoricaldata.net/ They're using Category Theory to inform their schema design and query language.

I read about OLOGs years ago and was very intrigued. Cool project!

I checked the origin of the project and it seems to be an European grant to a few universities working on Semantic Web. Given the track record of this technology I’ll pass my turn. Neither the home page nor the doc provide any compelling arguments or killer feature for it’s use over existing solutions.

I’m not convinced either that a strong schema is really important for a graph database. In fact, even in OOP long chain of inheritance have proven bad practice. I also found myself that working with flat-type (no inheritance) nodes and edges simplify the code a lot.

Is this database used on any deployed project?

You checked the origin as in you read my comment on this thread? Pretty clear when I said it started in Trinity College Dublin working with linked data. Semantic web certainly suffers from true believers that refuse any break with orthodoxy, but there is a core of wisdom & imho if you can do something practical the best ideas can emerge.

Without schemata, you end up with spaghetti pretty quickly and no way to ensure data-quality. It's also impossible to treat segments of the graph as documents. With a strong schema you can move seemlessly between objects and graph view.

It is currently deployed in several industrial settings.