Speculative bubble or no, perhaps the greatest challenge facing Bitcoin, cryptocurrencies in general, and the broader world of blockchain is the fact that all of these efforts in some way support criminal endeavors.
Read more here: https://www.forbes.com/sites/jasonbloomberg/2017/12/28/using-bitcoin-or-other-cryptocurrency-to-commit-crimes-law-enforcement-is-onto-you/#27a74be83bdc