The editable point has been made (workaround: make a new post periodically), but here are some other considerations:

• organization - at a certain point, a massive post is hard to navigate; a wiki should show splitting it into sub-pages (could link to other lemmy threads, but that kinda sucks to maintain)
• searchable - once it's in multiple pages, I'd want to skip the index and search for something directly
• linkable - I can link a lemmy post, but that link can bit rot if the pinned link changes
• edit history - sometimes advice changes, if you use git, you can explain why in the commit message without cluttering up the actual text; likewise, if we find new advice to be bad, we can easily undo it later since the old versions are still there (esp. if it's something complex like a flowchart)
• can be shared by other communities, like maybe collaborate with an investing community, or a community for financial advisors
• discussion - if advice is contentious, it can be discussed away from the main community, so meta posts don't clutter regular discussion (could solve with a separate meta community)

I think a separate wiki has a ton of value.