Other than the gaps in the airport data, the biggest benefit of using reanalysis dataset might actually be for radiation purpose so that the shortwave/longwave radiation values are derived from atmospheric physics model rather than empirical methods like the Perez model using cloud coverage. Airports cloud covers are actually reported by people, in increments of 8th (1/8, 2/8, 3/8 etc.) and originally intended to help aircraft.
For parameters like temperature also, airport data tends to be more noisy, as you can see in this comparison of EPW data from Whitebox Technologies for Boston in 2016. You see values of hourly rate of temperature change ranging from ~-10C to ~+8C for ISD data vs. about +/-4C for ERA5, which seems more likely unless it’s an extreme weather event.
FYI, NASA also has POWER LARC service providing reanalysis dataset based on MERRA-2 reanalysis data for building energy simulation uses. Dru and Linda from One Climate Building did some comparison of ISD, MERRA-2 vs. ERA5 dataset for about 30 cities around the world for 2004 to 2018 and the alignment apparently looked very good (on an initial assessment).