I've only used Ableton, FL and Reason, so there could be something I'm missing, but doesn't Reaper support vsts? How could the ecosystem for plugins possibly be weak?
VST/3/AU/DX, and it supports scripting your own in JS as well. Unless they're upset about not having LADSPA or Nyquist, I can't imagine what else they'd be referring to that isn't proprietary AVID garbage.