I semi-seriously dabbled in cryptocurrency last year — on the speculation side, not the mining side — and through blind luck managed to make some money off it, but I ended up selling all my holdings after coming to two conclusions:
First, cryptocurrencies are an absolute nightmare for the environment.
Mining a cryptocurrency is essentially trading electricity and time in return for a digital number that's hopefully slightly more valuable than the expense of the electricity and time required to generate it. But there's a huge invisible deferred cost that individual miners don't pay, and that's the long-term environmental impact of widespread cryptocurrency mining.
That's an incurred debt against the future of the planet, which will be paid by future generations. Any value derived from it now is essentially value derived by stealing life from future humans. I just can't get comfortable with that.
Second, even during my brief time casually dabbling in Bitcoin, Ethereum, and Litecoin, it became abundantly obvious that the cryptocurrency markets were being heavily manipulated by large-scale pump and dump operations, and that the markets in general were just rife with scams.
It felt like what I imagine it would have felt like to invest in the stock market before there were any financial regulations. Scammers and schemes everywhere, and lots of little people getting hurt. I didn't want to be a part of it.