Croatia is similar to Turkey in that sense. They are known to win against the biggest teams like England and France only to lose the game against some much weaker team. For example, at the start of the Nations league, they have lost the game against Austria 3-0. I don't consider Austria as a weak team, but I think that Croatia is much better team and losses like that should never happen. But everyone has a bad day once in a while. It's better to have a bad day at the start of a competition then in the final game 
If they finish within the top 2, then they can qualify for the main tournament. Only other team (apart from Turkey) who can spoil Croatia's chances is Wales. They are one of the most under-rated teams within UEFA and on their day they can make life difficult even for stronger sides such as England and France. But I don't know how they are going to perform this time, especially after the retirement of Gareth Bale. The other teams in that group (Armenia and Latvia) are there just to make up the numbers. During recent times, their quality has gone down and I don't think that they stand any chance against teams like Croatia, Turkey and Wales.