Annihilator Pods have a fire rate of 120 rpm, Flak Cannons are only 60 rpm, so it's no surprise that they cannot destroy the entire rocket stream. Flak cannons are solid against both bigger missiles (Harpoons) and tight missile groups (Locust, Swarmers, to some extent Harpoon Pods), but Annihilator Pods exist explicitly to screen enemy PD, especially things like single flak. Also, 3 Flak Cannons are only 24 OP, 4 Annihilator Pods are 40 OP.
3x Dual Flaks (36 OP) completely suppresses 4x Annihilator Pod streams without extra help. In fact, just 2x Dual Flaks (24 OP) does an excellent job of minimizing the damage an Onslaught's rocket stream does; a few shots might get through but overall the protection against that particular threat is satisfactory. This can be tested painlessly by slightly modifying an Onslaught loadout, taking it out against a SIM onslaught and swapping some PD weapons.
Of course, this lasts up until the Onslaught's other weapons knock out your PD guns, since the PD value of a disabled PD gun is 0. Flares can also complicate things, since even with IPDAI they'll force proximity fused weapons to detonate early and let the rockets get far more close than is comfortable.
As far as "PD Lasers are the best PD options" go, if I have a Hybrid slot and I need anti-missile defense, I have a few options depending on size and OP budget: Vulcans, Flak Cannons or Dual Flaks. Beam PD doesn't rank, Period.