The misnamed Mask of Vulcan is actually an over-sized, metal helmet that makes the wearer completely invulnerable to all forms of physical and (direct) magical damage.
However, the helmet is extremely large, heavy and ungainly. It is not padded and being completely invulnerable (cannot be drilled or heated), there is no way to equipped it with a practical chin strap. It sits square on the wearer's shoulders but the helmet does not bestow any added strength and its very heavy weight (approx 100lbs) makes it very difficult for most normal people to wear for an extended time, much less exert themselves or run. Tripping, falling or being bowled over is also still very much a concern, at which time there is 90% chance the helm will fall off.
The eye slits are small and the ventilation holes in are very small, causing the helmet to heat up rapidly when worn, as well as severely limiting the wearer's sight and hearing. The ventilation holes also make the wearer vulnerable to (non damaging) gaseous attacks and suffocation.
But otherwise the whole, total invulnerablity thing is still really cool.