As my opinion it's safe. Atleast safer than centralised Exchanges. If you have full access of your private key and phrase then there is no problem. Just use some popular Dex don't put your phrase or private key in a new Dex. And one more thing beware of scams. There is many Dex sites which is not original they are phishing sites of some popular Dex and if you trapped there that will be a real problem.
In general view, DEX is not safer than centralized exchange, that's very obvious, otherwise, DEX have already gain bigger volume compared to centralized exchange and we will see more coins trading in the exchange. DEX is for small traders only but those who trade with bigger money, they prefer centralized exchange as they can own a password, have their account with 2FA which is not possible with DEX as it's pretty risky to input your private key if you don't know how to minimize that risk.