Are Wi-Fi and app features on pellet grills actually reliable?

It depends on the brand. Traeger's WiFIRE app drew generally positive community feedback in the threads we read, while a high-trust r/pelletgrills thread flagged serious connectivity problems on some Z Grills WiFi-equipped units. The lesson from reviewers is that app control is a genuine convenience when it works, but it should not be the sole reason to buy a particular model.