Thats because people try to adjust them like a float style carb, its a different animal, but not too much unlike a float bowl design you mentioned. I have dealt with these carbs for over a decade. The diaphragm is the issue and once its understood, things can become easier. With a float style carb of this nature you can adjust the float level to make tuning more pleasant, with a diaphragm you are stuck with the stiffness (or how worn out) of the diaphragm to determine what would be equated to float height. Some people will keep on sticking in new diaphragms (as mentioned here before) to keep the easy tuneability. I have found that usually the diaphragm will only wear to a point and then kindas level off (to a point of failure) . This is the same as trying to tune a float style carb with a float level thats too low (but who would ever do that, when its so easily correctable). It makes the screw adjustments more complicated as the low requires more movement and the high less as with the low level because it creates a greater overlap of fuel between the two ranges. You have to go back an forth between the two "sweet spots" of high and low to balance out the middle. It can mean the difference between starting out with an 1/8 turn on the high equaling a 10% increase of power to 1/32 turn half that increase after the bottom is dialed in. The condition of your needles can add or take away from this as a rotted and pitted needle screw will obviously be more unpredictable that a clean and solid one. The presence of fuel will always add to the breakdown/wear of the diaphragms simply by gravity and this will be significantly increased with fuel that contains ethanol as it breaks down the diaphragms material much quicker (some aftermarket brands do hold up better that others). Then you have temperature changes that effect the diaphragms flexibility, in my experience the change is less noticeable with a worn diaphragm compared to a new one. You may ask why would any would want to deal with any of this, the answer is simple.... A float style carb has its limitation of angles it can operate in compared to what is being talked about.
