McKinsey in a recent report has nicely summarized some common misconceptions about the blockchain.
From an investors' point of view, point 3 is highly relevant. If more than 50% of network-computing power is controlled by the same party the blockchain network can be tampered (The so-called 51% attack). The blockchain is then compromised. The 51% attacker can issue cryptocurrencies at will, invoking the double-spending problem.
Source: McKinsey