It's a 2 frame window with a fairly precise analogue input. It's inconsistent, especially under pressure. A 2 frame window with a button input (i.e. a digital one) is hard but learnable; a 2 frame window with an analogue input both adds a significantly greater level of inherent inconsistency from input polling, and is a physically harder motion to perform consistently, and hence it is much more inconsistent. I thought I'd learned it well years ago until I actually started using it in tournament, and I still mess it up way too much.
Also when people are talking about shield dropping being hard they're often talking about the notch method, which is controller-dependent and outright doesn't work properly on most controllers (though incredibly consistent and in many cases faster than the non-notch method if you have a controller that can do it, which is why most pro players preferentially use it).
Also the non-notch method doesn't require you to tilt the stick in the direction you're facing. All you have to do is move the stick into the lower shield tilting zone for 4 frames, then beyond that into the lower area of the stick on frame 5 or 6 (the shield drop zone actually extends across a large area at the bottom of the stick, however it is normally almost entirely covered by the spotdodge zone; tilting the stick down disables spotdodging, allowing you to shield drop with the whole area for 2 frames, before the spotdodge zone disappears on frame 7). The particular motion of tilting is purely to make the execution easier; you can do it straight down too.
Compare these images:
Normal control stick map while shielding
Control stick map while shielding, after tilting control stick downwards for 4 frames
