With triples you need to reify hyper edges into nodes (a relation is just a hyper edge). This can lead to pretty awkward models, for example for temporal data. Most graph databases (eg Neo4J) have the same limitation and as a result struggle with temporal data or other higher-dimensional data.
We prefer to support general relations (hyper edges) and face the complications of query planning. We really want a general relational system that decouples the complications in query processing from the way you need to model your data and write your queries
This gets particularly important for what we do besides graphs: we also aim to support machine learning workloads (we have some references to papers on our website).
We still do automatic index selection, but indeed it is important that you do not create too many of these unnecessarily if you relations with arity bigger than triples.