Both can utilize automated systems and databases for known illegal content, but I'd think Reddit is just a little more difficult for such moderation as a lot of can be hosted elsewhere, while Pornhub content goes right through Pornhub.

Dunno what of outside content is scanned by Reddit when linked to, if it's just a thumbnail if it's a direct file link, or what, but that sort of issue can make it a little more difficult for Reddit.

In both cases it's not possible to catch all of it at submission, without manual moderation and verification for every single file coming through... which is technically possible, but no one would visit a site with content coming in that slowly. Really it's all on proving good faith effort in moderating it, and that's really all you can reasonably ask for in this day and age.