Ok, but you can't just base it on how much movement here is in the air/off the pitch.
No, but it's the key factor to model, imo. If the ball deviates unpredictably in the 0.15 window for a batsman with 0.15 reactions, it's functionally impossible for him to get it in the middle of the bat except by accident. That's a lot of explanatory power in terms of good and bad cricket shots.
Well, the obvious bit is that it's a good delivery because you physically can't get anything but the toe of the bat on it. If the physics is correct that aspect sorts itself out. Similarly, balls zooming down the legside, because any nicks are much more likely to run down to fine leg than generate catchable chances - partially because the keeper is the wrong side of the shot to sight it and there's no legside cordon.
The more complex / difficult to model issue with the yorker is that you have to move your head the furthest distance of any delivery to track the ball to the interception point. Really extreme shift to get a good look at your own feet, and v difficult to time eye/head movement correctly vs pace.
Or a short-ball at the ribs? That could be a great delivery on a fast/bouncy pitch but a not so great on a slow one.
On a slow, flat pitch vs a decent batsman it's a long hop. Rib high is perfect for the pull shot - but the thing about it is that you have to make contact with the ball early, with your arms extended in front of you.
Similarly, short balls from medium pacers are always long hops. It's always a crap ball to a decent batsman. Reaction time / predictability really is key here.
A length ball around off-stump might be great in the first over of a test-match, but is a terrible ball in the last over a T20.
And it's terrible-ness is largely explicable in terms of lack of deviation, isn't it?
The whole idea of T20 is that you bat in a way that won't usually work vs good bowling, so the pitches have to be doctored to remove this key aspect of good deliveries.
eta obv there's a further discussion about good wicket taking deliveries vs good containing deliveries.
Also, key aspect of good bowling is that you assess how much deviation is available - and whether that deviation is late enough to beat the batsman's reaction time - in picking your length.
If you know there's no movement and the oppo batsman must attempt big shots, a length ball is clearly a crap idea.