As opposed to mediawiki, where all citation data are stored as strings, Wikidata has a strict typing system. https://www.wikidata.org/wiki/Special:ListDatatypes
I think integration of citoid with wikidata would benefit from creating a wikibase specific export format. In particular, allowing:
* Possibly returning of integers in the JSON format for fields where this is expected (i.e.e number of pages) or at least validating the string can be coerced into an int
* Plausibly a structured date format compatible with wikibase dates. Failing that, validate all date fields and return ISO dates (currently return unvalidated strings which may be invalid for wikibase).
* Validating language codes against wikibase's allowed values
Other issues:
Wikidata expects isbns to have dashes (we don't).
Wikidata wants the pmcid without the pmc prefix.