Easy vs hard… definitely subjective.
When I speak about something being complicated, I specifically mean that it becomes impossible to think about X without also considering Y; the variables confound and complicate each other.
Having the database and web app on the same server does this: we can’t horizontally scale the web app, because we can’t horizontally scale a Postgres database; we can’t update the OS and run the app on it side-by-side to ensure stability before cutting over; etc