User blog comment:MisterWoodhouse/Volunteering your Fandom wiki for early migration to the Unified Community Platform/@comment-30737249-20200413111844/@comment-44807868-20200414133429

This should give you a satisfactory answer:

"Simple Fandom Wikis Stage. Once we are satisfied that the UCP is working correctly under real world applications, we will move into the next stage of rollout, in which we take 3,000 to 4,000 wikis and move them over to the new platform. This will allow us to see how pre-UCP wikis perform on the new platform. The wikis will be chosen by a mixture of volunteering from admins and staff selection, and selected based on the wikis complexity and needs to ensure they are setup for success on the UCP. If a wiki relies heavily on an extension we have not yet set up on the UCP, it will not be eligible for that transition."

- MisterWoodhouse

Based on it, I suppose the wikis which can experience blowback from UCP due to reliance or need concerning certain feature wouldn't be among the early migrated wikis.