First post for me. I have invested modestly in bitcoin and I want it to succeed. I am well read but I am no expert - and a total ignoramus with IT technical stuff.
I have read that if a large, rich state wanted to, they could 'hack'/break the system, presently and especially in the past . They have the '800m USD' of equipment (or whatever the figure was at that point in time) to do it without massive additional investment. So for ideological purposes and by a major power there may be a threat, but each day makes 'hacking' more difficult technically and more controversial/dangerous for that state. With govt sponsors like Goldman Sachs getting invested in BTC (albeit modestly), a hack gets less likely.
A real cynic may say "they" are interested in the technology and are using bitcoin as a live, wild field test to get people used to electronic currency - then later destroy BTC and replace with a centralised government backed/controlled copy. One thing you can be sure of is that governments would LOVE an electronic only currency only they control. But thats a concern for way further down the road and BTC doesn't threaten the dollar yet - it will be a lot more difficult to break the system by the time it does (if).
Hacking for profit by criminals is less of a concern I think, not only because of the investment in equipment needed (I believe currently about USD 800m - 20% of bitcoins 'market cap'), but also because if you fuck the system, you are basically tainting your own loot. To plunder in a subtle way would make the investment inefficient and to do so wholeheartedly would fuck the system and probably render the bitcoins useless or a lot less useful.
One commenter mentioned the transaction limits as the major drawback rather than security, and this matches my research. I understand there to be practical ways to fix this if the price of bitcoins gets higher, making the 'mining' more profitable. Obviously with more adoptions/transactions, the price has to rise wight he finite supply. However this is something I'm going to be reading about on here, as I don't really know how solid these 'fixes' are or if my thinking is straight.
Peace and prosperity!