1. Lethargy during a sideways market
2. Disregarding your strategy
3. Chasing somebody elses dream
4. Believing somebody elses FUD
5. Using money that you shouldnt be
6. Failing to properly do a self risk analysis before investing
If you can cope with these dangers then this type of investment will do well for you. This is a hands-off, long term method of investing. DYOR, find good coins with bright futures, buy and hold. Dont worry about the daily percentages
I don't believe in some of the point you mentioned above as that to me is a miss leading of people into making looses in their trading.
I don't think there is any denger in holding Bitcoin or some other top altcoins since most of them react positively with Bitcoin market increase.