Even 'conductive' ESD packaging materials have significantly higher resistivity than aluminum, so the way that discharges occur through these materials will differ. A more resistive material will allow a slower discharge, resulting in lower currents, which may be beneficial. On the other hand, a more conductive material maintains a lower potential difference across it when a given current flows. Hypothetically, if all pins of a device are in contact with something really conductive, like aluminum foil, then probably won't be much of an issue. But if some pins are NOT in contact, and a discharge is applied, then it's possible that the higher conductivity will allow a more rapid discharge through the device than an otherwise equivalent 'conductive' ESD material would and therefore more likely to result in damage to the device. In practice the details probably matter quite a bit. Since aluminum foil does pierce easily and also has no elasticity it seems very easy to end up losing contact between the foil and the device with vibration/jostling, it certainly seems like a poor substitute for a proper ESD shielding bag. But it's also probably better than nothing as long as you're careful about wrapping it.