Geodesic versus planar geometry

Global

Buffer, distance, and intersection operations computed on raw lat/lng as if it were a flat Cartesian plane diverge measurably from true great-circle geometry, worsening with distance and latitude.

Detection

Compare planar buffer/distance output against a geodesic (e.g. Vincenty/haversine) computation for the same inputs; diverging error grows with radius and |latitude|.

Mitigation

Use geodesic buffering/distance libraries (e.g. turf with units, or explicit ellipsoidal projection) for any radius beyond a few km
Document the max latitude/radius where planar approximation is acceptable

Affected conversions

Point+radiusH3
H3inscribed circle
H3circumscribed circle
Line/corridorH3