From my understanding, Pawn only compares one piece of gear to another one in a complete vacuum.
So let's say that for your class, 1 point of Haste is equal to 1.5 points of Crit. It will tell you that a piece of 100 Haste is better than one with 149 Crit and worse than one with 151 Crit... Problem is, from my understanding, that since it compares pieces in a vacuum, Pawn doesn't take into account that you may be currently wearing 1000 of Haste and 0 of Crit (which would make change the ratios significantly, and now 1 point of Crit may be better than 3 points of Haste for you).
I was also unsure if Pawn was to be trusted, and this is when I discovered they suggest you to use SimCraft and stuff like that to get new values when you change some pieces of gear, to keep Pawn update with your current real ratios.
Also, there are probably bugs, since Pawn keeps telling me any green staff is a 250%+ upgrade to my epic one... My wild guess is that Pawn thinks I can equip a 2nd staff in the offhand or some shit like that.