Just want to share, I can't prove that mnemonic safe or not. I'm not an expert for things like that.
It's safe there are millions of wallets users that rely on Mnemonic to open their wallet, if it's not safe then we have hundreds or even thousands of complaints and users will discard these wallets
My friend just got hacked, lost about 300-400 USD.
He wonder how it happened, make some probably the device was infiltrated by malware, if yes. All his key is gone(all), so there's no only 1.
Malware and interaction with a project are just two of the reasons but doing nothing is just impossible.
He was generated Mnemonic by coin98. Never connected to MetaMask or some things like login to connected, never.
Your friend needs to dig deeper he might interact with one project and not remember it or use an infected PC to open his wallet, I'm not a user of Coin98 but there's hardly a scam accusation on Coin98