I have not used those phone based light meters / PAR meters.
They’ll probably produce sufficient accuracy for what an individuals needs. It will also likely differ from phone to phone and have other limitations since the sensors, filters, gain, linearity of the CCD/CMOS camera will vary. Some applications may spell out the phone models that the application has calibration data for. There was a thread here talking about using such applications.
For more accuracy (if you a running a large operation) or scientific grade accuracy (if you are running studies or tests), you’d want a true PAR meter or, in some cases, going as far as a spectrometer.
p.s. a true par sensor makes adjustments for the incident angle of the radiation. The angle relative to the sensor will cause an error. Correcting for the error is essential for accurate metering of PPFD for instance. You don’t get that from a phone camera but, then again, do you need that level of accuracy.