Arsenal cannot show similar performances in the matches they play. This makes it difficult to predict Arsenal's matches. I thought the Newcastle game would be very difficult for Arsenal, but it wasn't like that. Arsenal took the lead 2-0 in the 24th minute. Newcastle could not show much presence on the road and conceded 4 goals.
I guess you did not watch how Newcastle played in their last few matches. Newcastle played inconsistently this year, they conceded so many goals in the last few matches. They could not even win against Luton few weeks ago and conceded by 4 goals. If you knew how Newcastle played (at least in the last few matches), I believe you will not say that Newcastle will be hard opponent for Arsenal. The odds for Arsenal for the match (1.35-1.38) is also a proof that Arsenal is much more superior than Newcastle.