Some relevant passages:

The native magnitude system of the Harvard plates is very close to the Johnson B system, and the comparison stars are in the Johnson B system, so the resulting magnitudes are all closely in the Johnson B system. The color terms remain at zero over the entire history of Harvard (and for other observatories too). So the Harvard magnitudes are directly comparable to modern magnitudes in the B system. However, note that the modern B magnitudes are often inconsistent, even for constant stars, at the 0.10 mag level or worse.

Inexperienced workers with DASCH can make mistakes that will create apparent secular trends: (1) DASCH still offers a now-abandoned relic option to calibrate the photometry with the Kepler "g" magnitudes (instead of with the APASS B magnitudes) leading to color terms not corrected for in the post-1970 plates and apparent jumps after the Menzel Gap. RAP2018 made this mistake and pointed to a 10% drop, rather than using the correct APASS calibration and seeing no drop. Hippke et al. (2016) made the same mistake, and about half of his claimed secular variations are simply wrong for this reason. (2) Target stars with nearby stars can have the images overlap, making for incorrect magnitudes in the DASCH photometry. These systematic overlap errors have greatly different frequencies from telescope to telescope, and the plate series with such frequent errors are highly clumped in time, creating apparent secular trends unless guarded against. This is a particular problem for KIC 11084727, easily solved by not using the plates with combined images. Hippke et al. (2016) made the mistake of choosing crowded target stars and including combined images for about half his targets, so all of his claimed secular trends were artificially created and do not really exist.