Can't the PF not lift-off and not downgrade anyway? Even in ideal circumstances, the best this strategy should ever be able to achieve is a draw.
In order to catch him by surprise and not get your landing blocked by an enemy worker, you would have to land quite a ways away from your target, at just the ideal range. Even if you're successful at killing the opposing player's nexus, you now have like 5 workers and a PF out in the middle of nowhere. As soon as the protoss player recognizes he can't kill the CC before it upgrades, he can just withdraw, let his nexus die, and use his army to deny the terran's mining. In the end you would have a grounded PF out of position, and the protoss would have a random building or two but plenty of workers (and probably a zealot or a cannon) to kill any SCVs who attempt to mine.
Also, this should never work against a terran or a zerg. Zerg overlord scout should see the CC coming, and the terran should be able to upgrade his own CC to a PF to defend himself (if he's already completed an OC, it's probably too late anyway).
Additionally for this to work, close positions are required. You would have to make a big sacrifice just to scout early and see if this is even remotely an option. If he's not a close position you've waste an early worker, and there is a chance he'll see your early scout and know you're up to something.