Unfortunately, the RDFa output in core is unreliable. The following issues outline the ways in which it breaks and possible solutions.
Known bugs
#1777688: RDFa output incorrect when not using entity template (Views, Panels, etc) or when render array is altered
#1778034: Field formatter developers can easily pollute a site's RDFa output
#1778194: RDF module can't handle compound fields
#1778410: Throw exception when RDF namespaces collide
#1228872: RDF default mappings override empty values
#1780090: Decide: Use RDFa Lite's property attribute, or give users choice
Improvements
#2034951: [META] Support and test all field formatters RDFa output
#1778122: Enable modules to inject attributes into field formatters, so that RDF attributes get output
#1791992: Split term mapping functionality and RDFa output into separate modules
#1869600: Refactor RDF mappings to be inline with the new Entity Field API
#1791678: Change the way RDF mappings are stored
#1784234: Change notice: Use schema.org types and properties in RDF mappings
#1813328: Enable literal handling in common RDF-ish domain models
#2020001: Make datatype callback handling more flexible
#1323830: Place title RDFa metadata inside entity HTML element
Clean up issues
These are issues that will probably go away as a result of the work, and should be closed or attended to once it is done.
#1166140: Allow modules to put RDF hooks in modulename.rdf.inc
#712914: rdf_template_variable_wrapper does not work for unrendered template variables
#621086: Module-defined mappings are not deleted on uninstall
Comments
Comment #0.0
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded namespace issue.
Comment #0.1
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded default mapping override issue.
Comment #0.2
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded RDFa Lite 1.1 decision issue
Comment #0.3
Anonymous (not verified) CreditAttribution: Anonymous commentedAdd #1791678
Comment #0.4
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded #1791992: Split term mapping functionality and RDFa output into separate modules: Split term mapping functionality and RDFa output into separate modules.
Comment #0.5
Anonymous (not verified) CreditAttribution: Anonymous commentedReorganized
Comment #0.6
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded #712914: rdf_template_variable_wrapper does not work for unrendered template variables
Comment #0.7
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded #621086: Module-defined mappings are not deleted on uninstall
Comment #0.8
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded #1166140: Allow modules to put RDF hooks in modulename.rdf.inc, and moved small issues to the bottom.
Comment #0.9
geerlingguy CreditAttribution: geerlingguy commentedAdded issue #1338966.
Comment #0.10
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded #1784234: Decide which types and properties from schema.org are relevant for core
Comment #0.11
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded #1869600: Refactor RDF mappings to be inline with the new Entity Field API
Comment #0.12
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded 1778122: Provide a way for modules like RDF to inject attributes in Twig templates
Comment #0.13
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded 1813328: Enable literal handling in common RDF-ish domain models
Comment #0.14
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded 2020001: Make datatype callback handling more flexible
Comment #0.15
Anonymous (not verified) CreditAttribution: Anonymous commentedRemove #1338966: Introduce _rdf_mapping_load_multiple() to reduce queries
Comment #0.16
scor CreditAttribution: scor commentedadd #1323830
Comment #0.17
Anonymous (not verified) CreditAttribution: Anonymous commentedAdded 2034951
Comment #1
Anonymous (not verified) CreditAttribution: Anonymous commentedBased on the current status of the work, I am recommending that we switch to an alternate approach... #2152459: [Policy] Deprecate RDF module and move it to contrib
Comment #10
quietone CreditAttribution: quietone as a volunteer commentedComment #14
smustgrave CreditAttribution: smustgrave at Mobomo commentedThis can probably be closed out now
Comment #15
longwaveExactly ten years after it was opened, I agree this can be closed out now - the remaining child issues that are relevant have been moved to the RDF contrib module now RDF is gone from core.