Panathinaikos is in a pretty dreadful state.Really a shell of their former shelf.Their last championship was in 2010 and iirc they have 2 championships in the last 15-20 years(although to be fair the league is corrupt).The last few years they have struggled,finishing 2nd-4th,but they at least got the Greek cup a couple of years back.However,every single year they humiliate themselves in Europe,getting knocked out by smaller teams without playing good football.
Right now,i am not 100% familiar with their current roster,but i bet there's nothing to fear.Marcus Berg is their star player,but he is worse than a few years ago.Still,one of the very few players that can make a difference.Other than that,the club is in a bad economic state,which means that any transfers they made probably aren't that good.The coach is fairly new and the fans don't like him,and generally only a few players have been in the club for more than 2-3 years.
In other words,i don't know if Brondby is a good team,but i would be surprised if Panathinaikos gets through.If they do,i expect that they are going to get humiliated in the group stage.