Strange but according to the wiki you linked...... They have been caught mistranslating statements numerous times, see above links.....
The accuracy of MEMRI's translations are considered "usually accurate though occasionally disputed and highly selective in what it chooses to translate and in which context it puts things,
As for selectivity, yes they only quote high level Palestinians when they make nasty statements. That means that some high level Palestinians make nasty statements. Of course you would never criticize those statements, only MEMRI for daring to publicize them.