H.I. Sutton did a great video [1] about it that also explains how it was beneficial due to the way enemy submarines had to estimate speed and heading and could get fooled.
When you're trying to hit something moving 20kt, with something moving 30-35kt, from a few thousand yards, it doesn't take much error in estimating speed, heading, or distance, to make them miss. It's honestly more remarkable that they hit at all in those conditions, even with a "spread" (shooting several along slightly different headings hoping one or two will hit).
Anyone with a copy of Silent Hunter 3 https://store.steampowered.com/app/15210/Silent_Hunter_III/ can experience this for themselves. Trying to estimate another ship's heading from a vantage point near sea level is maddening, even with the aid of a little recognition book which shows ships of that class at various relative headings, and as TFA says it's especially hard to tell the difference between perpendicular and near-perpendicular relative headings.
Mind you, this also explains why dazzle was a promising idea in the first place. Calculating Angle On Bow was already the hard part of ship-against-ship targeting, why not try to make it even harder?
SH3 vets plotted relative positions to get heading much more reliably, which was something that was also insanely difficult for sub captains.
With some trig you can get relative headings by taking bearing measurements, waiting 30s, and then taking more, but speed must be known.
To get full heading and speed you can do the same thing multiple times but now we're in a least squares problem and it has plenty of singularities.
The most reliable method for me was to pass in front of the ship submerged the fire tail tubes or turn around if you have enough time.
Insanely difficult for fast moving targets.
I played so much SH games from the first days on, that I went and did my PhD on tracking using bearing measurements (albeit mostly applied and only a tiny theoretical contribution).
Well, at least in WW2 they had mechanical computers that could use the input width of the vessel and the class to estimate the range, heading, and firing angle to set the torpedoes up. There's a good series of youtube videos by a sim player that teaches how to use the TDC: https://www.youtube.com/watch?v=ANk6hZCcVRw - it's very in depth, and demonstrates how much effort it takes to get a firing solution, as you say.
The worst thing is that the TDC's calculation is very much GIGO if you get the Angle On Bow input wrong, and that's the hardest thing to get right.
Even with a spread, you're really just narrowing the odds, not guaranteeing a hit
Adding to the difficulty is the possibility of observers seeing the incoming torpedoes and a fast-moving, maneuverable ship dodging them. Fast and maneuverable as in .. the USS New Jersey, able to sidestep a spread of five torpedoes from a Japanese destroyer.
https://en.wikipedia.org/wiki/Japanese_destroyer_Maikaze#Con...