Using any device like a phone, a tablet, or a computer running windows or macos is a risky thing to do.
ANY online site is even worse.
They all have major security issues about your information and what you type into them.

An offline computer, running ubuntu, would be one way to securely generate a wallet:
Ensure the wallet is created with a pass phrase, dump the keys (and be sure you have the private keys all written down/printed correctly) then wipe the computer HDD.
(don't use an SSD, since most people don't understand how to properly wipe an SSD)