Ethereum uses complex mathematical algorithms in order to create digital currency and maintain the network. There is a massive amount of computational power backing the Ethereum network, making it very difficult to perform attacks on Ethereum’s consensus layer itself.
Most of the risks associated with using Ethereum come from using decentralized applications (dApps) that are built on top of the platform. Since Ethereum is a completely decentralized platform, anyone in the world can create an Ethereum dApp and deploy it to the mainnet. In other words, there is no quality control, since nobody can prevent an Ethereum dApp from being deployed.
Since transactions on Ethereum are permanent and irreversible, you don’t have much recourse if your funds become inaccessible due to a smart contract bug, or are stolen by malicious developers.
So, while the Ethereum platform itself is very secure and works as intended, the same cannot be said for all the applications that are deployed on top of it. Before sending your funds to any Ethereum-based dApp, make sure to do your own research and that you’re dealing with a trustworthy application.