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]

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