It is safe to use and one of the most easiest options out there when it comes to holding ethereum and bunch of ERC20 tokens on ethereum blockchain. Here you take full responsibility for your coins and you must keep your computer secure and safe to use. Don't store your private keys/mnemonic in plain text, yes people often do this and it's not best in practice.... Try to keep your coputer clean from viruses, you may want to use some linux debian or redhat based as there are not that many hackers that write special hacks for those Operating System distributions, this doesn't mean that you are safe here by default but will have higher chance of less viruses/hacks that could potentially be there to steal your coins.

As other stated here hardware wallets are one of the safest things out there when it comes for security, since your private keys don't leave your device. They are easiest option out there for ordinary people and for tech ones. But they are not necessary tool in order to keep your coins private.

Coinomi is not open source + it doesn't have support for many tokens that are there on ethereum, other thing is that I trust them and use their wallet since it is convenient for storing multiple alts but not tokens. Since many people use android/IOS devices I wouldn't agree that they are safest place either for keeping your coins. Safest is cold storage when you are sure that you just want to put some coins away/hold/not use them.