The one constant contraindication for a composite restoration is
The constant contraindication for using a composite restoration is the inability to properly isolate the operating area. Occlusal wear of composite is similar to that of amalgam. Extension onto the root surface may result in gap formation with composite but also results in initial leakage with amalgam, indicating that there is no ideal material for root-surface extended restorations. A high C-factor (Class I) can be largely overcome by using

a liner under the composite,
a filled adhesive, and
incremental insertion of the composite.
Occlusal factors.
Inability to isolate the operating area.
Extension onto the root surface.
Class I restoration with a high C-factor.