I was looking for complex .eps files and ran across this one. It contains an .eps, .ai and .svg version of the same art. I opened all three versions in Affinity Designer.
The .svg is kind of ugly as it's never been a great vector format. Especially not for complex art. The .eps is really ugly since a lot of unsupported transparency is lost. The .ai file is, of course, perfect.
But here's the odd part. PDF/X-4 isn't all that old, but it does not export the .ai file very well. A lot of the transparency areas do not convert well, despite the fact this format supports transparency. I tried changing a lot of the preset choices, but nothing helped.
If I use the first choice, PDF (for print), it's a perfect match to the .ai file. But then, that isn't really all that surprising since all Designer has to do is export the .ai file out as is and give it a .pdf extension. You can copy the .ai file and just change the extension to .pdf yourself and get the same result.
PDF/X-4 export:

PDF (for print) export:
