Thanks everyone for the answers.
But after all, Bitcoin mixers are a third party, like exchanges, where are the guarantees that they will not rob me?
If you are using a centralized mixer, there are no guarantees. You have to trust them to not scam you. That's why you want to minimize the chances of getting scammed - if you ever need/want to use a mixer - by using a trusted mixer.
You can also just use CoinJoin to mix your coins, since they are, AFAIK, trustless and even got recommended by theymos, through Wasabi, in
this post.