Unless the whole process can be done offline, i wouldn't use the service/tool, it's simply too risky for minor convenience.
Besides, when we're talking about convenience, IMO it's more convenience to use multi-cryptocurrency wallet with 1 backup (either mnemonic seed or wallet file).
I don't see any security risks here, you're not generating any private keys with this trick, and it only uses publicly available information. Bitcoin public keys can already be recovered from blockchain data if you spent coins from that address, and you can't generate a private key for a known public key.
Also, for the purposes of this method signing a message would work too, since you can also recover a public key from that signature too. So, you can generate Ethereum address even if you didn't spend coins on that BTC address yet.