What is a forex card, and is it better than your debit card abroad?
A forex card is not cheaper than your debit card. It is more predictable, which is a different thing and sometimes worth more.
One is fixed in advance. One is decided while you are asleep.
What you are actually buying
Three things get bundled together and it helps to separate them.
A fixed rate. You convert once, on a date you choose, and the rate is locked. Currency movement during your trip stops being your problem. This is genuine value if the amount is large or the trip is long, and close to worthless for a weekend.
No foreign transaction fee per swipe. Debit and credit cards typically add a percentage on every foreign-currency transaction. A forex card has already converted, so it is spending in the local currency and that fee does not apply.
A spread taken up front. The rate you were given when loading was not the mid-market rate. The margin is in that rate, which is why "zero fee" appears in forex card marketing so often and means so little. You paid, you just paid once and invisibly rather than repeatedly and visibly.
Where it goes wrong
Unspent balance. Converting back is a second conversion at a second spread, and some cards charge a fee to do it. Money left on the card after the trip is the most common way a forex card ends up more expensive than a debit card.
The wrong currency. A card loaded with euros used in a country that does not use euros triggers a cross-currency conversion, at a rate that is usually worse than the one you were avoiding.
Dynamic currency conversion still applies. A terminal abroad may still offer to charge you in your home currency. Say no. The forex card has already done the conversion, and accepting means paying for it twice.
Fee stacking. ATM withdrawal fees, inactivity fees, reload fees and reissue fees vary enormously between providers and are where the real difference between cards sits.
The honest comparison
For a short trip with modest spending, a good no-foreign-fee credit card is usually cheaper and always simpler, and it carries stronger dispute rights.
For a long trip, a large planned spend, tuition or relocation, a forex card's fixed rate is worth having, because the thing you are buying is not a lower cost but a known one.
And the comparison that settles it is the same one that settles every currency question: ignore the fee structure and ask how many units of local currency you end up with for a given amount of home currency, all charges included. One number, at the end.
Packing sandwiches for a journey. Not because sandwiches are cheaper than food, but because you decided the price in your own kitchen rather than at a counter you have never seen, at a moment when you are hungry and have no alternative.
Where you meet it
Every bank offering you a travel card three weeks before a trip. Every leftover balance sitting on a card from a holiday two years ago. Every terminal abroad asking whether you would like to pay in your own currency.