The tax issue obviously depends on where you live. In Denmark there is no tax on cryptocurrencies (lucky bastards!). In Norway, where I live, we have to pay tax on every transaction. Even when buying cryptocurrencies with other cryptocurrencies. This is hopeless. But using the TenX card doesn`t make it worse. It might make it easier, even. Because after trading hundred of times between cryptocurrencies on exchanges, by the next tax return, I have to trace every single trade and calculate loss/winnings. I am not looking forward to this. However, if I instead would just buy ETH with fiat, send it directly to my TenX wallet, yes, I would have to pay taxes on every transaction when using the card, but then I would easily know how much. I would just have to add all expenses by the next tax return and whatever amount exceeds my original purchase amount, I would have to pay taxes on (I am assuming they will have a way of seeing your total monthly or yearly expenses here, like normal credit cards do). I hope this makes sense.
Also, you should read this. The real benefit of the TenX card: