why a custom built converter isn't for everyone....

Exactly what I mentioned earlier. People call and tell their build, and most of the time one of their "off the shelf" already built converters is close enough to fill the bill and that's what they ship. People get all shook up because they want to think every turbine was fully implemented with their build, and only their build in mind. So many builds are so similar that the need to weld every tooth on the ring gears especially for Jon Doe's car is ridiculous.
Maybe one way of looking at it is that you are calling for a “recommendation” not so much “custom” Some companies like the two I mentioned above have a slew of stalls available and actually list them. Other companies do not, so you call for that recommendation and they say you need a 3800. It’s one they make often, but it’s not really “custom” is it?