Without custom domain: Growth Suite Playbooks, Promoted Smart Links, and all other Advertising campaigns will only be able to report back on the # of link clicks happening on your ads. This may exclude important info on everything that occurs after viewers click on the link. Prior to the iOS 14 changes, users were able to optimize campaigns for conversion events (ie higher value than a link click) without needing a custom domain attached to their campaigns. Running link click campaigns, which don't explicitly bid for higher converting individuals, means you may need to spend more to reach relevant parties.