My experience with graph DBs: schemas usually become implicit and ad hoc, minimal tooling support and even less mature tooling, difficult to explore and understand your data, performance is often poor, memory usage can be problematic because they usually pull the graph into RAM, and there are few people with real experience (comparatively to traditional DBs) which can create a negative feedback loop.

I have yet to run into a use case where a graph provided more value than a relational model. I'm sure they exist, but I haven't found them yet.