I see. Maybe to come back to Mor’s question about our definition of usability, which I would derive from definitions of “data quality”: According to literature on data quality, it can be defined as intrinsic (quality of the data themselves, including aspects such as data accuracy and consistency), accessible (relating to technical and cognitive accessibility), contextual (quality of data for a specific context and need - including comprehensiveness, timeliness and disaggregation) and representational (structure and syntax, version control, existence of metadata for data attributes, etc.).
I assume all of these elements could be seen as factors enhancing or limiting “usability”.
Given that these definitions are sufficient to delineate questions of usability, I would argue that usability/quality aspects relating to data structure, accuracy, consistency or comprehensibility of data (values) are all hard to measure - partly for the reasons @Stephen and @Mor already mentioned (crowdsourced, laypersons can fill out survey).
I think that your proposal, @Stephen, seems to be an interesting proxy for usability measured through the existence of supporting metadata. Do you have an idea how we can define the right metadata for that? We could ask for descriptive metadata for data attributes that inform about strengths and weaknesses of the data, the existence of metadata documenting how data are collected, metadata documenting update frequency and versioning, etc.