It's probalby just the cheap Promax calipers. I have some of them, and some cheap noname ones, and some cheap Chinese-brand-name ones I forget the name of right now, and all of them are the same in their lack of adjustability, finickiness, and lack of braking power unless the pads are adjusted so close that they rub if the whole thing isn't exactly perfect. (this is on my CrazyBike2 custom-built semi-recumbent).
They are what they are--cheap.
OH, and it also makes a great deal of difference which levers are used. If the levers (or cable!) has flex or stretch, the brakes will take a lot more pull than they should, forcing you to adjust them closer to get full braking before the handle hits the bars. IF the lever is the wrong type, with the wrong pull ratio, they'll also ahve the same problem.
FWIW, most of my rim brakes work as well as the cheap disc, or sometimes better, when neither is kept perfectly adjusted before every ride, as long as the rim is true. If the disc are perfectly adjusted they seem to have slightly better braking but it can take an hour for me to fiddle with them enough to get them "right", then 10 minutes of riding will put them right back out of adjustment. Not worth the time; I keep them on there only because I don't have *good* disc calipers, and with the weight of CB2, I need more braking than rim alone can provide. (so I use both at the same time on the same wheel, and regen braking on the rear wheel).