What are your thoughts on cryptocurrency-backed Credit Cards?

I wrote a piece about the crypto-backed MCO Visa before: https://milelion.com/2019/10/22/why-i-wouldnt-t... The tl;dr version is that it's fine if you're already into crypto and want a way of spending it more easily in the real world. But if not, you shouldn't be getting into crypto because you want the credit card. The benefits on offer could easily be negated by a small movement in crypto prices (which are notoriously volatile), plus you could get things like 0 FCY fees with products like youtrip/revolut, and lounge access with $30K credit cards (not unlimited, but you can get multiple different cards).
Rewards simply don't justify the risk.

(below I use BTC but consider it a reference to cryptocurrencies in general)

Not too sure what you mean by the credit cards being "backed" by BTC - there is no instance I know of a mainstream bank offering deposits/loans (of which credit cards would be an unsecured variant) in BTC.

What is available is accounts where you put in some BTC and then use the card networks to spend. It is a nice innovation as the early promise was BTC it could be spent on real-world goods/services but has got increasing difficult given the mining fees and need to have confirmations etc etc.

The companies setting up these cards are essentially offering a normal prepaid CC with backend magic to convert your spend to BTC. Do note that as part of the fees you may get a poor exchange rate to fiat when using these cards (in a similar way to banks give rip-off rates for non-SGD transactions on local credit cards)