What bad things did you read about CO?

You have to clarify what sort of Y fares you'll be flying on. Cheap discounted ones or the more-expensive ones? CO and NW give you 150% EQMs (but only 100% award miles) for more expensive Y fares. FB only gives 0-25% miles for the cheapest fares.

Lounge access: you get this E+ level and whether this is starts at top or mid tier depends on FFP and sometimes which country you're domiciled in. Lounge access is only for same day international flights (connecting on or from). Some airlines are more liberal lounge access but not by much. FB gives you NW lounge access when flying NW, even on domestic flights. If you can use an east Asian address, NW gives you complimentary WC membership at the Gold level.

If you can demonstrate earning status, CO and NW may comp you. I do believe AC E is (or was) only 35k miles so they may not match you to mid tier.

Most ST airlines give you a loyalty bonus for flying on alliance airlines unlike most *A FFPs.