Purify didn't get here until late last summer and it took him quite a while to get acclimated. He was much higher rated than Pierce is coming out of JUCO. Frantz Hardy was a 4 star coming out of Butler with offers from NU, Auburn and some other big programs.
The parts in bold mean nothing. That is not to say both are not talented but rankings mean zero.
True, never know what we'll get w/ JC's and, true, going to D-1 is a shock but there are plenty out there who acclimate and get it done. While Purify was not getting it instantly he was playing pretty well by early in the Big 12. James Terry did well right off the bat for us a few years back.
As for Brown not playing receiver in high school....not a big deal. Plenty of high school QB's and RB's end up playing receiver in college. Two notable were Anquan Boldin and Quincy Morgan.