Just want to share, I can't prove that mnemonic safe or not. I'm not an expert for things like that.
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.
He was generated Mnemonic by coin98. Never connected to MetaMask or some things like login to connected, never.
Usually i think that Mnemoic phrases are very safe. I don't really know the numbers but as far as i know it is almost impossible to brute force the because there are millions of possible solutions obviously. So i would guess the reason why your friend was hacked was not the mnemonic phrase but the way he created it or how he saved or stored it. Usually you are connected to the internet when you are creating your mnemonic phrase no matter if you are creating it for metamask or for a desktop wallet. So if your PC is already infected with a malware or something when you are doing that they can already get the phrase there.