It depends both on what mixer you use, and how you use the mixer.

First of all, there are a number of mixers which have been proven to be breakable. There is a great write up about this here: https://bitcointalk.org/index.php?topic=5117328.0. The only mixer that user was not able to break was ChipMixer, since they use a unique mixing process when compared to all other mixers.

Secondly, it also depends on how you use the mixer. If you deposit a unique amount of bitcoin to the mixer, let's say 1.376 BTC, and then you immediately withdraw the exact same amount of coins. Someone could reasonable assume those two transactions are linked. If you send your mixed coins back to an address you've already used, or you combine your mixed coins with unmixed coins somewhere down the line, or you send your mixed coins to somewhere you've completed KYC, and so on, then you can completely negate all the benefits the mixer brings.

Best practice when using a mixer is to deposit a set amount of coins, and then slowly withdraw different amounts at different times to different places, and make sure you never link mixed and unmixed coins together.