Are Amazon-listed 'professional 15-in-1' air monitors trustworthy?

Specialist communities are skeptical. Threads on r/AirQuality repeatedly warn that many cheap multi-sensor units use a single low-cost MOX element and derive most of their 'parameters' (CO2, HCHO, TVOC) algorithmically from it, producing readings that look precise but aren't independently calibrated. PM2.5 readings on these units are usually the most defensible number; treat the rest as directional.