Where were France, Croatia or Belgium?
I think France and Croatia can be excused for not putting the effort in, especially in the early games.
Both teams were filled with players who played long seasons, some winning their leagues, some the CL, and then the WC final where one team left elated and one disappointed - but I imagine both equally empty.
I can only speak for Croatia, the boys were not ready for the Spain game (6-0 destruction), physically or mentally.
They also repeatedly downplayed the competition in the media, publicly favoring old friendlies where you can test players free of pressure and have your main players rested... Playing England in an empty stadium didn't help either.
They only looked more interested for the last 2 games, but by then they'd dropped 5 points and it was too late (even when they led 1-0 at Wembley, the team was dead tired and never seemed in control of the game).
So, IMO, I think the competition can be the "League Cup" of the internationals... It's more competitive than friendlies and I imagine if you do get to the semis you may as well try to win it.
There's a lot of pros there.
Cons? Two that stand out IMO.
- seeding system is too longterm... Imagine Germany wins the WC and the next EURO, but they start the next Nations League in Division 2 - meaning they can never be a top seed - ridiculous.
- no rest for the top players if the game matters at all... Guys like Rakitic and Modric (speaking just of the Croatian guys) desperately need that rest - but they didn't get it this time around