Which material is commonly frowned upon for modern wiring due to safety concerns?

Aluminum without proper treatment is often regarded with caution in modern wiring applications mainly due to its propensity for oxidation and the potential for loose connections. When aluminum wiring is improperly installed or not treated, it can develop a thin layer of aluminum oxide. This oxide layer is a poor conductor of electricity, which may lead to increased resistance, overheating, and ultimately a significant fire hazard.

Moreover, connections made with untreated aluminum can expand and contract with temperature changes, leading to a phenomenon known as "creep," where the metal can loosen over time. This loosening can further exacerbate the increased risk of arcing and fires. For these reasons, most electrical codes and safety regulations emphasize the need for either copper wiring or properly treated aluminum for safe electrical installations.

The other materials listed, while they may have their specific applications and considerations, are generally considered safer or more acceptable under current standards when correctly utilized.