> If joins are a critical performance-sensitive operation, the most important property of a DGGS is congruency.

Not familiar with geo stuff / DGGS. Is H3 not congruent because hexagons, unlike squares or triangles, do not tile the plane perfectly?

I mean: could a system using hexagons ever be congruent?

Hexagons do tile the Euclidean plane perfectly. They are the largest of the three n-gons that do so.