Adding autoplay videos that are tangentially related to the wiki page.
Said videos take about 10 seconds to show up and will cause the screen to scroll to a different point from where you are currently reading
Their privacy policy is exhaustive, and you opt in by default to sharing your data with Google and 7 other analytics providers
UI noise like "share to Instagram/YouTube/LinkedIn" that probably 0.1% of people even consider using
When you're on a specific game wiki, you care only about that wiki content, but fandom places a Fandom navigation bar and side menu
Their default email strategy sent almost daily updates for random posts on (unrelated) FANDOM community when I made a couple comments on a specific wiki a decade ago.