A study which was conducted using OpenBCI Cyton V3-32 to evaluate the performance of TI ADS1299 against a high quality lab based system. Conclusion: the performance of the ADS1299 in combination with wet Ag/AgCl electrodes is analogous to that of a laboratory-based system in a low frequency (<40 Hz) EEG recording.
Full text: Available on ResearchGate
Also see this previous report by Jeremy Frey,
https://hal.inria.fr/hal-01328427/document
Jérémy Frey: Comparison of an open-hardware electroencephalography amplifier with medical grade device in brain-computer interface applications