What are the biggest risks at holding a massive amount of hbd?

The blockchain dies or the price of $HIVE goes to zero. Outside that, it is a very low risk.

I'm thinking about converting some fiat into hbd......and what is the best to that?

Depends upon how much you are looking at converting. The liquidity is tight right now. If a relatively low amount, buy HIVE and then trade it on the internal exchange. Might take a bit of time to get your orders filled since liquidity isnt great there but it will get it done over an afternoon or so.