What Is a Major Security Risk Associated with Proof-of-Work?

The major security risk is the 51% attack. This occurs when a single entity or group controls more than 50% of the network's total hash rate.

With this majority, they can prevent new transactions from getting confirmations, stop other miners from mining, and most importantly, reverse their own transactions, leading to 'double spending.' This attack is extremely costly for large networks like Bitcoin due to the required hardware investment.