Your only real problem would be if the spot gold fell, then you WOULD lose the spot value of the coin you have paid and you MIGHT lose the numismatic premium you paid if there is a rout in the gold market if fear took over.
EG. The Perth Mint was selling the 1oz gold dragon for $1,860 today and gold was about $1,770 at that time. Ignore post cost etc you are paying about $100 over spot for this coin.
IF gold sank to say $1,500 a coin you would probably still be able to sell your coin for $1,600 ie. your numismatic over spot is maintained @ $100 and you only lose spot.
But if gold went to say $800 you might find there arent too many buyers for your gold around at all and you might have to sell closer to spot of $800 thereby losing your numismatic premium.