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

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?
Put your armor of steel on, they are coming for you now... LOL. Now QUALITY of converter is huge....