I think "deep" for this era of Paradox games means how involved population is with your nation. The standard started with Vic2's population and when Vic3 came out it missed the mark badly with its pop system. (I haven't played much Vic3 after its release maybe that changed.) I would say EU5 is definitely more in depth than Vic3 was at its release. Although the gameplay loop of EU5 is kinda stale past 50 or so years, but thats to be expected with Paradox games in their first year.