It's a quite tough question. Because the risk of investment on bitcoin is depends on the site which you are use to investment.thank you.
I wouldn't say here is the biggest risk in the site where you hold your bitcoins, because there are not only sites to hold your founds, there are such things as hardware wallets and paper wallets where bitcoin is 99% saved.
The point is bitcoin is high risk investment and this risk goes only from it's high price. Imagine you invest in bitcoin 10k$ to by 4 BTC (by today's price), this is big money to loose, right. And what is the loose? You will loose when the price suddenly drop and never raise high back again.
I hope I explained good what is the thing about risk of investing in bitcoin. But I truly doubt in some huge fall and bitcoin's death. I think it will climb upper and upper for years till will be not replaced by some more technological crypto-currency.