We have the same issue of not always knowing which rendering is the best to use, but be aware that my solution is not going to suit everyone. We have been users of QPDF since v7, principally for rendering PDF to TIF, but also for creating and editing/annotating PDFs. In the early years there were often 'edge cases' where the rendering was incorrect, mainly for CJK documents, but also some others; Debenu were usually able to fix these in the next release, but not always. Most of our users do not control the PDFs which are to be rendered, and silently creating incorrect output was an unwelcome feature. So PDFIUM seemed a safer choice. As you have discovered, PDFIUM is more likely to support all the edge cases.

As a result, since 2016 our default rendering solution has been PDFIUM, using the DLL packaged for Delphi by Winsoft, which we found just before Debenu released a PDFIUM option in v13. We stuck with Winsoft because they seem to add support for new PDFIUM DLL versions more frequently than QPDF, but if I was choosing today I would probably use the QPDF PDFIUM option.  Yes, for both solutions there is an extra DLL to deliver and update, but we can manage this. 

We continue to use QPDF for all PDF applications other than rendering; it has many more features and is much easier to use, and we have no reason to abandon our existing code to call PDFIUM directly. And we fall back to regular QPDF for rendering in several situations: (1) for applications where we have to render PDF documents using multiple threads in the same process; (2) after PDFIUM returns an 'unsupported' error or raises an exception; (3) as an option for users who have grown to prefer the QPDF rendering for their specific documents.