All is hackable, above all if it is a online system.
But i think that is not easy to hack the bitcoin system, and the team is constantly work to make secure their systems
Ask that question of the 1,000's of people who have lost 10's of millions of USD, think mt-gox or ethereum faux wallets
There are many elements of hacking BTC,
1.) fake web sites that look real, which harvest private keys ( most common easy way to make $5 Million USD in one afternoon )
2.) mt-gox type infiltratin of exchanges where private-keys are held, another easy way for 'hackers' to gain access to millions quickly
3.) the mother of all of course is breaking the sha-256 hash, "Google Brain-Flayer", its been done, but that gig is over, there are many advanced NSA tricks for breaking ECDSA, which is how FBI can any time it wants get the private key for any address ( think silk road BTC auction )
4.) the theory of experts is that NSA created BTC to wait and see when the PUBLIC would/could break SHA-256, I speculate it will be broken by public within 2-3 years, before that GOV will send out a policy statement and tell all GOV to use SHA-512; banks will be kept on easy to hack stuff forever, and they'll keep public on SHA-256 as long as they wish
5.) Large Bitcoin Collider - this project if it gets 1k, or 1m users, all targeting their asic/gpus farms towards any specific address say 'satoshi1', then selective hacking will be gold, and just like mining pools, people will be paid a share of the profit as high value coins are hacked,.. How soon? Again it just depends on how quick he can get ppl aboard.
Eventually BTC will update itself to SHA-1024, so this is all no big deal, for safety keep your wallet in cold storage and printed, keep your BTC's off of these 99% fraudulent exchanges that just churn your BTC's for "FEES"
Lot's of people working on hacking SHA-256, I know the NSA can do it, and I suspect that folks 'private' in Israel are doing it on as-paid basis, just a matter of time before we get a little better HW, and sometime discovers a new ALGO for SHA-256, in general what the public see's is 15 years behind advanced industry and GOV, FYI in 1968 Tooley published "FFT", but Standard Oil discovered it in 1956 and was sharing with NSA, 17 years it took before Academi caught up with adv-gov-tech