Something is only a sound investment if you know all sides are bound by rule of law. If you were to put money into a shell game at the local unlicenced market, then that would not be classed as a sound investment (even if you did know how to "beat the system" there's a good chance an accomplice would mug you for your winnings after you walked away). Or if you received a cold call from someone claiming to be a stockbroker with "insider knowledge" encouraging you to buy shares that they "know" are about become very valuable, that would not be classed as a sound investment.

With cryptocurrency it is the same. The combination of zero consumer protection and anonymity is the perfect breeding ground for fraud. If you were being charitable, you could call converting fiat money into cryptocurrency a gamble, but certainly not a sound investment. The only people who will try to convince you otherwise are those set to gain from the fraud.