my opinion of the various wristblades in the movie is...
that the longer "alien hunt" wristblades were acid proof (although there's no evidence for or against this), but I like this idea none the less...
and the shorter "human hunt" wristblades are not acid proof, hence we see them melting in the film...
each pred had a unique "uber" weapon... Scar had the longer wristblades, Celtic would have had the biggest plasma caster (if they were'nt taken) and chopper had the super blades...
so celtic only had the "human hunt" wristblades to take on Grid, and they were'nt too effective...