[Sohcahtoa82]

Using smart contracts, people are creating cryptocurrencies within an Ethereum network. Sort of a blockchain-within-a-blockchain. Right now, Polygon is a popular alternate Ethereum network for doing these.

The problem of course, is that once these contracts are published onto the network to create your new cryptocurrency token, it's very difficult (perhaps impossible?) to update them. And of course, if someone finds a vulnerability in one of the contracts and exploits it, it's game over. There's no reversing the transfer of tokens.

So to give a more explicit answer to your question, it's not a problem with Ethereum, but a problem with the contracts people are writing on the Ethereum network.