For me it's having some fandoms that aren't easy to locate on FFN. To use an ineligible example, Critical Role appears as a "world" in the Dungeons and Dragons category in the Games section, rather than in the Web Shows category in Misc, but some people don't realize that and post in the Web Shows category anyway. Because CR is classified as a "world," I can't further filter down to other "worlds" that may be eligible (like the various one-shots/standalone spin-offs, other unrelated series from the CR team's programming, etc). It makes writing eligibility posts for non-straightforward fandoms a huge pain.