Very interesting. Non-disparaging clauses would be deemed unenforceable in many countries where statutory consumer protection legislation already applies. Those that generally vent their frustrations publicly have already exhausted vendor cooperation & are desperate for an outcome. Public disparagement works, & I'm not afraid to call a spade when I need to where there is blatant misrepresentation, merchantability or fitness for purpose issues. Much quicker & cheaper than going to a tribunal or court. As a consumer, gather your facts, leverage CCTV, use your phone camera & don't get done over by the many carbolic smoke-ballers out there.
I would hate to see online public forums disappear because of participation paranoia due to growing fear of libellous litigation risk.