The Collaborative International Dictionary of English v.0.48:

Simplistic \Sim*plis"tic\, a.
   Of or pertaining to simples, or a simplist. [R.] --Wilkinson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

simplistic
    adj 1: characterized by extreme and often misleading simplicity;
           "a simplistic theory of the universe"; "simplistic
           arguments of the ruling party"

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "simplistic":
   arrested, backward, coarse, conceptually crude, crude, embryonic,
   in embryo, in ovo, in the rough, intellectually childish,
   oversimple, reductionistic, reductive, rough, roughcast, roughhewn,
   rude, rudimental, rudimentary, stunted, unblown, uncultivated,
   uncultured, uncut, underdeveloped, undeveloped, unfashioned,
   unfinished, unformed, unhewn, unlabored, unlicked, unpolished,
   unprocessed, unrefined, untreated, unworked, unwrought