Cost is low but 3rd party ecosystem including plugins is weak. It s like notepad++ of programming editors or winamp of music players. Effective, cheap, non innovative Yet OK.
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.