In my experience over the years, and in reading many posts here, it seems that many builders make great whistles in Eb, C, Bb, etc. but their D’s seems to be the weakest of the bunch. For example, how many of us have played a great Generation Bb or Eb, but never a great D?
Is there a reason why good D’s are harder to make than other keys?
Tres
No
More generally, I think certain whistle designs have a “sweet spot” which sounds best. For instance, one of my buddies who spent a good deal of time visiting Glenn Schultz’s workshop observed that Glenn’s design sounded best when the tube was a certain diameter compared to the length.
In his wood whistles, this wasn’t a problem – he made the “ideal” whistle to match the design in every dimension. But for the Water Weasels, he was limited to using ready-made plumbing pipes, so generally each size of pipe had an ideal whistle key to go with it.
So for the narrow width, the C whistle was always weak, while the Eb and D were nice – I think maybe Eb was considered the best at that width. On the larger, grey whistles, Bb is a bit better than A; on the next size up, G is clearly superior to F. My friend told me B-natural was a problem – too short for the grey tubes, too long for the small ones.
In general, I suspect it is true that any time you used a fixed width of piping for several different keys of whistle, some keys will sound better than others.
This is very true. I didn’t mean to be (too)flippant with my earlier one word response.
The ratio of diameter to length has a LOT to contribute to the quality of a whistle’s sound. For a given diameter, if you get too long, the sound gets weak and wants to jump into upper overtones too easily. For this reason, tabor pipes typically have a long length to bore ratio. Similarly, make the tube too short and the upper register is unstable or impossible to play. Sure would be nice to make all keys of whistles from one tubing diameter, but you can only tweak the physics just so much…
A really good way to understand this is to get a set of whistles with multiple bodies and a single head. (I have a Syn D-C-Bb-A set, but the Susato 3-body would work. A Walton’s Mello D and Walton’s C pair works too - same head and bore).
Even with the same head and bore diameter, the different bodies can sound noticably dissimilar. On my Syn set, the D is loudest, most responsive, and requires the most push to reach the upper register, while the A body is quieter, less responsive, and has a stronger upper than lower register. As you’d expect, the C and Bb bodies fall in between the two extremes.
Exactly what I was going to say. There really should be a compiling of this information so I know which whistle to get in what key. 
Exactly! Note that to some degree, you can compensate for the differences in body length by voicing the whistle differently.
I hate these long-winded roundabout responses!
Tres
I wouldn’t have thought that high D’s are harder to make because they constitute most of the market and my experience is that there are lots of really good ones around. I have noted however that certain makers tend to have trouble with some keys (not ever high D with the better makers) and seem to shine most in one or so keys. I’d say more, but I’ve already far surpassed the standard Busman word count.
Philo
Some whistle makes seem remarkably consistent across the range of common keys. Overton in particular come to mind here. Others do seem to be especially popular in certain keys. Just how fair this is I don’t know; we are often influenced by the choices of popular players and, naturally enough, by whether we ourselves got lucky with a brand in a certain key.
Since someone was bound to start a list, here is a list of whistles that seem (to me) to be especially good in certain keys. In drawing up the list I’m leaving out high D and I’m not really meaning to imply that the whistles I choose are not good in other keys but just that this is where I found a certain magic. I also leave out Overton (there isn’t one I don’t love) and cheapies.
High F: Abell
Eb: Sindt
C: Burke AlPro
G: Copeland
F: Grinter
Low D: Copeland
Low C: Howard
For A, Bb and low Eb I would go for Overton and I should say that in only picking one key I’m probably not being fair to Sindt. But this exercise is pointless if you aren’t highly selective.