Advice was structural steelwork a last resort for an earth electrode:why?

Probably a lack of understanding, or at least the uncertainty of how well the steel is connected and how well it would handle fault currents. Also if the depth is unclear, the performance in dry weather may be variable.

But, if it is designed in from the outset for steels to be used as an earth, with well designed connections,  such as welded on earth studs, then steel piles or rebar in concrete can be very effective, especially if the concrete is seeded with marconite to make it more conductive - a floor slab is then one large area conductive plate.

I'd be more wary of trying to drill and lug to random bits of tinplate that you don't know if they may not be well connecting. The key is knowing something about the building construction.

I have had  single figure ohms off a marquee before, that was 'nailed' to the ground with multiple steel pins at the base of each upright.

Probably a lack of understanding, or at least the uncertainty of how well the steel is connected and how well it would handle fault currents. Also if the depth is unclear, the performance in dry weather may be variable.

But, if it is designed in from the outset for steels to be used as an earth, with well designed connections,  such as welded on earth studs, then steel piles or rebar in concrete can be very effective, especially if the concrete is seeded with marconite to make it more conductive - a floor slab is then one large area conductive plate.

I'd be more wary of trying to drill and lug to random bits of tinplate that you don't know if they may not be well connecting. The key is knowing something about the building construction.

I have had  single figure ohms off a marquee before, that was 'nailed' to the ground with multiple steel pins at the base of each upright.