I agree that Croatia aren't one of the teams to be underestimated at all. First of all it is true that they haven't been much successful in Euro tournaments recently. They couldn't even get past 1/8-finals twice in the most recent tournaments. However their performance in the recent two World Cups has been very impressive. First the finale and this time semi finals.
With the player pool they have it isn't impossible for them to have a good success in a Euro tournament also finally. There are really skilled Croatian players now and my expectation level from them has increased by their nice work in the latest World Cup.
They just need to break a leg finally by Euro 2024. They have the potential to play for the finale.