Basically, if two players of equal skill end up in a PvZ...the zerg will win if:
1) He displays IMPECCABLE control of his macro (i.e. timings, injects, creep)
2) He creates PERFECT unit compositions to "counter" the protoss units
3) He creates PERFECT, zerg-favorable engagements
4) The protoss screws up royally on at least one of the above aspects and/or does not use his spellcasters
I don't think you understand the match-up. Zerg isn't about "perfect" unit combinations, it's about using cheap units (that are very cost efficient except against large tier 3 Protoss armies) to keep map control and develop a superior economy. This allows the Zerg to work up to a Hive tech army that Protoss needs either a large well upgraded air fleet or Collossi/Stalker/Archon with good control and a great Vortex to engage cost efficiently. Even then, the Protoss often loses unless he can also uses warp prisms and DTs to keep the Zerg expansion count down.
Before Hive tech, the Zerg can either play aggressively, trying to knock out the Protoss third and do drops or muta harass or else macro and turtle behind spines. The Zerg should be able to reactively build an army that can maul anytihng Protoss builds as long as you catch it in the middle of the map until the the HT, archon, carrier, VR, or Collossi count reaches critcial mass. At that point, Zerg switches to BL/infestor or Ultra/bane/infestor. The Zerg then either tries to crush the expensive Protoss army and wipe out bases to end the game or else turtle to a max corruptor/BL/infestor army while turning more and more drones into spines as the bank gets bigger, to create an army the Protoss simply can't engage without a lucky vortex (giving up the mobility advantage for power except for a cadre of infestor/ling for backstabs).
But "perfect compositions"? Roach/infestor is viable versus everything until late game, and muta/ling is also good against anything but a double stargate opening until late game. And late game is usually corruptor/BL/infestor, which is viable against everything. And while good infestor control and good splitting is required against good opponents, it's still easier than HT/blink/vortex control. It's Zerg macro, setting up surrounds, and multiple point harass where Zerg gets interesitng and challenging. Even there, Zerg timings are more forgiving than both other races. It's keeping up injects, spreading creep, spending larva, knowing when to build units and when drones which is hard, and that's reactive. So Zerg requires more map awareness, more APM, especially more macro APM, and also more unit multitasking than Protoss. But it's not harder, per se. It's just different, requiring different skills. Play wha tyou like better.