When would you blend data rather than join it upstream?

Blend when the data genuinely lives in different systems that cannot be joined — ad spend from one platform against conversions from another.

Join upstream whenever you can. A blend queries each source separately, so it is slower, it is limited to five sources, and the join keys must be dimensions.

The most common blend failure is keys that look identical but are not — different casing or punctuation on a campaign name produces null metrics with no error. Normalising the key with a calculated field before blending is the fix.