Unless they are fairly large the anti siphon holes can and will eventually get clogged over time. I used to have them on my return nozzles to break siphon and had to increase their size a few times until I reached about 5mm I think. Even that clogged over time with various algaes
Anyway, as mention already you can guarantee their effectiveness by regularly checking them. I on the other hand got fed up with that added maintenance. It was just "one more thing" to pay attention to, maintain and support on what is already a complex system.
Anyway, just thought I'd throw that out there.. You know, since Mr. Murphy is such a big fan of out hobby and all
In the end I just let them clog up for the last time and instead built a bigger sump to support the extra 3 gallons to siphoned out from the return nozzles.