Non-ferrous implants may be considered unsafe if they are:

Non-ferrous implants being mechanically or electrically activated can pose safety concerns, especially in environments such as MRI (Magnetic Resonance Imaging) or other electromagnetic fields. When such implants are activated by external forces or currents, they may cause adverse reactions in the body, such as overheating, movement, or even malfunction of the implant, which can lead to complications or injuries.

In contrast, factors such as the weight of the implant, whether it has been machined, or its age do not inherently imply a safety risk in the same way. Heavy implants might be cumbersome but are not necessarily unsafe due to their weight. Similarly, while machining might affect surface properties, it doesn't directly relate to safety when exposed to magnetic fields or electrical currents. Older implants could raise concerns about material degradation, but the essential risk factor in this context is the activation mechanism rather than age or weight.