Transaction mode has one real footgun: SET statements are session-scoped, but in transaction mode your session gets reassigned between transactions. Easy to miss until you're chasing a mysterious search_path bug at 2am.

psycopg3 prepares statements by default now, which breaks in transaction mode unless you explicitly opt out.

For apps with a persistent server process and an in-app pool the external bouncer is mostly ceremony. The math changes with serverless: no persistent process means no persistent pool, so a dedicated pooler starts pulling its weight.