Both of Monaco vs Barcelona and Atlanta vs arsenal are not a huge surprise as many people are portraying it. Atlanta were the Europa league champions beating last season Bundesliga winner Leverkusen. Monaco too is a second team in french league last season. Both of Atlanta and Monaco were playing at home. Manchester City vs Inter Milan was much surprising for me. But its first round of league and there are plenty of matches to be played.
Yes that is right Recent surprises in European football like Monaco beating Barcelona and Atlanta beating Arsenal should not be shocking. Atlanta won Europa League against strong German team Leverkusen and Monaco did well in French league last season. They also had advantage of playing at home. What really surprised me was Manchester City result against Inter Milan. But let's not forget, it is just the beginning of the season and teams have many more chances to recover and change the outcome.