To me, I do not think holding altcoin is dangerous except if the coin you have is not a promising one but if it is, it is very profitable for holding because the price will grow and become higher by the time you decide to sell and you will be able to make reasonable profit from it.
Considering if we invest in the major coins which have good marketcap, we won't be feeling worries too much. Some altcoins have their potential in making profit. The thing is whether we choose the right altcoin or not.