Salzburg, just proved they shouldn't be underrated. One of the problem of big teams in the champions league is that they underrated small teams and sometimes this teams used that to their advantages
Other than the fact that coming up against a smaller team in the UCL can make the bigger teams feel complacent, another big issue is the fact that most times in the UCL, you're playing a team from a different league, an unfamiliar team that you've probably not faced before or if you have, some years back.
It makes the match tougher as it'll be difficult to understand their style of play. Liverpool are used to playing spurs, Chelsea, Newcastle etc, so it'll be easier for them to gameplan this teams as they know all their strength and weaknesses, but against Salzburg, the coach would really need to watch clips to be able to come up with a winning strategy.
To me that's what makes the champions league fun and tough, when teams square off against unfamiliar foes and must have to adapt to come out on top. Liverpool did well to win last night.