The two big ones are:
Unlike a fiat currency which is backed by a Sovereign Government, a Bitcoin has NO backing.
Liquidity: A grand total of 18.6m Bitcoins have been mined since inception; far fewer than that are in active circulation.
There is a conservative estimation that 20% of all Bitcoins mined have been irretrievably lost in one way or another and that 95% of the ones in circulation (or not in circulation would sum it up better) are held by just 2.9% of all registered addresses.
This has two effects: with there being such a small supply on the market at any one time you end up paying a sizeable premium; if that is not bad enough just wait until it comes to selling.
On January 2nd this year a sale totalling 150 Bitcoin resulted in a 10% drop in the price.
It was noted at the time "If you can destroy the market like that in the space of seven or eight minutes, that shows there is no liquidity and no depth — nobody is there to take the other side of the trade when things start moving. You have these extreme moves because everyone is on the same side."
More than 2000 Bitcoin wallets hold over 1000 coins in them; can you imagine the carnage if just one person attempted to offload their position?
My advice would be to get the fuck out while you still have a shirt on your back; you might think its great craic getting all trendy with the Millennials but they have nothing to lose.
Sell your position and send me the proceeds; I will get you in on the ground floor for the next big thing; you will be well ahead of the curve.