It is not dangerous if you hold potential coins. I think holding alts would have more risks than BTC, ETH and so on because no one can predict the price will go up or down and will fluctuate a lot due to trading market. you'd better hold some of top 10 coins and become quite profitable.
I find that holding alts can only be profiting when you have them as trading assets, otherwise, they can only be useful if they have credible use in the mainstream event like a utility token. But in all, most people happen to hold alts because they envision that another 'BTC' would emerge.