Adam commented on marketplaces here & it doesn’t sound too promising (since I expect Monzo will expect Freetrade to pay a referral fee if new users sign up to the service having found it through Monzo) -
there are some situations where I can imagine marketplaces offering a lower acquisition cost / access to a large group of new users, which might make being part of them more viable but those are hypothetical scenarios right now so I haven’t asked about them 
I expect this questions going to come up a lot as Monzo, Starling (maybe Revolut) & even in high street banks build marketplaces so I’ve moved your post to a new topic, to make the discussion easier to find, I hope that’s ok!