There isn't very much lock-in other than the domain names - Fandom wikis are all built on MediaWiki, and they have all of the MediaWiki export infrastructure (which exports everything about a page, including history) still intact last I checked.
Wikia/Fandom has a history of refusing to delete wikis where the community has agreed to migrate (such as when many wikis left after the forced skin change in 2010), leaving a stale copy with better SEO. I don't know whether they still do this nowadays, but after that incident I swore never to contribute to Fandom ever again (for new wikis I try to recommend other wiki farms, usually Miraheze).
Fandom's lockin is mainly not being able to migrate your community (they will ban you if you link to the new community) and having more SEO juice than you.
Someone will copy your wiki's content into Fandom and Fandom will starve you voa SEO.
If that's really happening, then DCMA take-down?
Most Wikis use permissive licenses. Also, has anyone actually