Are metal NFC cards usable?
Yes, and only because they are built around the problem rather than ignoring it. Metal cancels the field, so a working metal card either has a non metal window over the antenna or an isolating layer beneath it. Both cost range, and both make the card one sided.
Why this needs explaining
A conductive sheet in an alternating magnetic field develops currents that produce a field opposing the original. Put a tag flat against metal and the field it needs is cancelled where it matters.
So a plain metal card with an inlay inside it does not read at all. The failure is total rather than marginal, which is why the existence of working metal cards looks like a contradiction.
They resolve it by construction, not by a better chip. No chip overcomes a cancelled field.
Two constructions
A window in the metal. The card is metal except for an area of plastic or resin where the antenna sits. The field passes through the window, the metal never covers the coil, and the card reads from that region.
An isolating layer. A ferrite or equivalent layer sits between the antenna and the metal, redirecting the field so it is not cancelled. The card can then be metal on the reading face, and the antenna works through the isolation.
Hybrid cards, metal on one face and plastic on the other with the inlay near the plastic side, are a variation on the second and common in practice.
What each one costs
Range, in both cases. A metal card reads at a shorter distance than an equivalent plastic card, because either the antenna is smaller to fit a window or the isolation absorbs part of the field.
Orientation, in both cases. The card reads from one face and not the other, and a person handed one for the first time will present the wrong side about half the time.
Weight and cost, which are usually the point rather than a drawback. And repairability: a metal card with a damaged inlay is not something anyone opens.
What to test before ordering
Both faces, with the oldest phone you can find. A card that reads on a current flagship and not on a three year old handset will disappoint half the people it is handed to.
The reading position, since a windowed card reads from a specific area rather than from anywhere on its surface. If the sweet spot is small, say so in whatever accompanies the card.
The card in a wallet, against other cards. Metal cards sit next to contactless bank cards, and the interaction is worth knowing before a thousand are printed.
And the engraving, if the design calls for it, since removing material from a metal face near the antenna area is not always harmless.
Frequently asked questions
Why does my metal card only read one way round?
Because the construction puts the antenna near one face and metal behind it. The other side is shielded by design, and that is normal rather than a defect.
Do metal cards read as far as plastic ones?
No. Either the antenna is smaller to fit a window, or an isolating layer absorbs part of the field. Expect a shorter working distance and test it.
Can I make an ordinary card metal by adding a plate?
That destroys it. A metal plate against the antenna cancels the field, which is exactly what purpose built cards are engineered to avoid.
Does a metal card interfere with my bank card?
It can, by disturbing the field in a wallet. Test the actual combination you carry rather than assuming either way.