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.
This is why I try to stick to anything that's established and has what looks like a bright future. I wouldn't find a coin that just came out today and expect it to be around for another 2 years or more. There's just way too many times it wouldn't be. If you're going to be holding long term, then try to stick to the top 50 coins or so.