The number of citations shown in RG for a manuscript for which I am a corresponding author is missing a lot more than in the WOS data, how does RG guarantee such accuracy?
I share your disappointment and frustration that there is such a large discrepancy. I think the most important deciding factor is what your organisation takes the data from. For example, mine uses mainly WOS so we have to obey that. My organisation prefers WOS for many reasons, as listed on their website.
It depends on the quartile and impact factor of the journals. Among different sources (WOS, Scopos, RG, Google scholar, ...), WOS is the most strict one. About WOS, it only counts citations of a publication that was published in impact factored journals. However, it doesn’t matter for Google Scholar where your paper is cited (with or without IF). That's why the H-index in Google Scholar is always higher than WOS or Scopos. On the other hand, it's assumed that the main source of RG is Google scholar. So it's not surprising if your H-index in RG be the similar as Google Scholar! While WOS only uses reliable sources, such as direct contact with authorised journals and their data. There is also an slight superiority for WOS rather than Scopos. This is why organisations prefer to consider WOS as their main source for assessing their researchers work.