PDFs on PDF version 1.4 (and possibly below, untested) may not convert properly. You can (crudely?) upgrade versions by running the PDF through online tools and adding an annotation, removing a page, etc.
If multiple sections share a page, that page will appear more than once in the output. This is a deliberate compromise; the alternative is mislabeled PDFs and nonuniform balance of sections.
Sections in the outline with the same name will break.
The outputs .zip file of some (all?) PDFs are reported as corrupt by Windows. I do not know why. Try 7-Zip to unzip if this is the case.
Metadata and other information is not preserved from the original PDF.
PDFs may become (a lot) bigger after being split. Like a 35 MB PDF might become 500 MB. Maybe. Sometimes.
Various other edge cases and weird behavior.
This tool also prints page numbers to console (hit F12) if you intend to use a tool like Stirling-PDF.
Split level refers to how many levels the output should have, maximum.