I would say PDFs are actually reasonably consistent though. Weird things happen on occasion, but I've certainly had more success than with SVGs.

They are reasonably consistent because there is a de-facto reference implementation (Adobe Acrobat) which, if your implementation does not match exactly, users will think your implementation is broken.

There isn't such an implementation for SVG.

We live in a world where Adobe set the standard, and anything that didn't render like Adobe was considered "incorrect".

You definitely don't understand PDFs, let alone SVGs.

PDFs can also contain scripts. Many applications have had issues rendering PDFs.

Don't get me wrong, the folks creating the SVG standard should've used their heads. This is like the 5th time (that I am aware of) this type of issue has happened, (and at least 3 of them were Adobe). Allowing executable code in an image/page format shouldn't be a thing.