I guess that's the thing about making whistles by hand - usually a labour of love by people who are not necessarily trained instrument makers or sometimes not even trained musicians; each instrument will be uniquei
I was talking about substantial changes to the design/voicing.
A slight variance from one instrument to the next can be expected. My advice here is always to try the whistle you are going to buy and if you can test drive several by the same maker, pick one that suits best.
However, design changes aside, I think a maker who can not deliver a high degree of consistency in their output has no business charging the prices you would expect to pay a skilled craftsman for their instruments.
I would also avoid makers who are poor players like the plague.
With my Syn whistles, I ordered a D/C set early on and then some years later the full lowA to high F set and was impressed by the quality (of workmanship and sound and tuning) having improved remarkably)
The early ones were pretty rough. I had one, sold it because I didn't like it and later heard Earle sent the man who bought it a new head to go with tubes he ordered to make a full set because he (Earle) thought the original one I was sent wasn't up to scratch.
My experiences with whistle makers have been mixed, to say the least. And made me wary. I keep an eye out, out of interest, but am not tempted to buy anything: as I said nothing I can buy is going to make me sound any better