Yeah, pre-1500 is rough. WikiTree is all about the sourcing and we all know not everyone has sources for their purported lineage. For more recent individuals it's fairly easy for anyone to source them if necessary but for earlier profiles, it's much more difficult. Even then there are some dubious connections made and it's a real task to verify links.
My best advice, if you want to go for the certification and add pre-1500 profiles, is to make sure the profiles you are adding now are all well sourced, especially the pre-1700 profiles.