fine in the stand but not on the trail?
Theres other things that can make a brand new brake seem poor surrounded by older kit such as:
at the rear:
lack of facing on the frame.
a poor bracket used (im thinking those cheap steel brackets that pop up on peoples bikes from time to time - ive seen problems solved once they have been replaced by a quality bracket)
a cup and cone hub with play in it.
a sealed bearing hub with worn bearings.
a hub that is a bit on the skimpy side that can flex around under hard use.
at the front:
same hub probs and the lack of facing.
legs moving independently under hard cornering with more body weight up front? (more so on usd forks perhaps? not quite sure)
bigger diameter rotors up front showing up the problems a little more so than the smaller ones used on the rear (picture the deflection at a radius of 80mm compared to a radius of around 100mm - if a rotor rubbed the pads at 80mm then its going to piss you off if its a further 20mm from the centre of the hub if you upgraded to bigger discs!).more so with cheaper cup and coned hubs.
All new brakes when fitted correctly run sweet.i cant think of a certain brake out there that consistently runs poor out of the box from new.they test them and put them into production and they all perform really well
..when fitted correctly.