When an engine has been sitting in one place long enough that the water is still or not rocking in the boiler I would agree that a properly maintained water glass is more accurate than a gage cock. When then engine is in motion the ability to accurately identify water level is skill the engine crew has to have a good feel for. An engine with a water column is easier to read as the rocking motion of the water is dampened through the column.
A few years back say 30 or so I worked as a teenager at Lakeside Amusement park in the Denver area. At that time the little Cagney Locos, forgive the spelling, had steel plumbing on the water Glasses and no monthly cleaning as required by FRA. The result was very sluggish movement of the water in the glass witch most of you guys know can be a bit of a problem. I got used to using gage cocks or tri cocks exclusively as I didn’t trust the water glass. I find them as easy to use as the glass. You can hear a very audible sound difference when a gage changes from steam to water. When the water is rocking in the boiler you can judge the level by the timing in the differing sounds and knowing what of the 3 is open. Although you do have to leave it open a little longer than most do to here the changes for a good judgment.
When I was at the C&TS I would have to switch the yard without a fire man from time to time. I found this advantages as you needed to be watching the movements and signals rather than the glass. When the gun was running you could crack the middle gage cock and listen to it to know when you should shut off the gun. When firing on the road I would use the glass on the column
Try-cocks should have a spud that reaches past the back head to help guard against the problem with water climbing up the sheets and giving a false reading.
Jack