I spoke to a member of EHG, the developers of Last Epoch about how the item faction system's Favor and Reputation gain work for the Circle of Fortune and the Merchants guild. Alongside my own testing here's what I learned!

Build in Video:
   • LAST EPOCH: Falconer Acid Flask Trapp...  

LAST EPOCH GUIDES:
   • The LAST EPOCH Beginner's Guide:  The...  
   • LAST EPOCH: Endgame Monolith of Fate ...  
   • LAST EPOCH: Can't Decide What to Play...  

Dr3ad's Guide to Merchant's Guild:    • Last Epoch Merchants Guild And How I ...  

Get Last Epoch on the Official ZIGGYD Indie Games Store & Support Ziggy & the Devs Both
https://www.nexus.gg/ziggyd/last-epoch

Timestamps:
0:00 - How Favor Gain Works
1:06 - EXP & Favor
2:24 - How Much Favor Does Different Content Give?
4:23 - Gimme the Data
4:45 - Favor Gain vs Time Spent
5:49 - What Surprised Me
5:55 - Full Clearing for Favor?
7:07 - Low Level Content is Maybe a Bit Too Good
7:52 - Other Small Tips
9:00 - Use Responsibly, Don't Optimize Away the Fun
9:42 - Reputation Gain Explained

Music by Le Matos
https://lematos.bandcamp.com/