Is Fisher's exact test always preferable to 2 test?

Nope! It depends if you can conduct the exact 2 test.

If yes, don't worry at all and keep using the exact 2 test all the time.

If no and you can only conduct the asymptotic 2 test, then Fisher's exact test is preferable only for 2x2 tables. In that case, the Fisher's exact test is equivalent to the exact 2 test.