There are many downsides to cryptocurrencies. These differ from one currency to another, one aspect may be the downside of one cryptocurrency but may be the positive aspect of another crypto.
You just need to judge for yourself, through experience and expert views about the various pros and cons of each cryptocurrency and decide for yourself which currency do you want to invest in.

For the ones who are here for some more detailed answers, one of the major downsides of cryptocurrencies is the fluctuating value it has. As you can see for bitcoin, the crypto once hypes and increases in value, but after that hype, the crypto is followed by a huge dip which may be stretched for too long.

Another negative aspect of cryptos are the bans and negative publicity they receive, many countries have been very slow to accept the importance of cryptos and this might be a huge negative aspect for some of you. Other aspects include the inability to use cryptos for normal day to day transactions or the lack of knowledge.