I don't have an opinion on whether understat or other sites are better for xG, but we don't really need it to see that our attacking output is lacking. We also don't need to go 5 years back and evaluate whether Allegri was good at building an attack back then. Fact is we have scored 50 goals this season which puts us 10th in Serie A in goals scored. obviously something is seriously wrong in this department, we should not be scoring so little with the players we have.
imo Allegri's approach of giving the players freedom in the final third doesn't pay off here, we don't have intelligent, creative players who are suffering from having to work within a system, we have the exact opposite type of players. Dybala may be crafty, but he's a selfish asshole most of the time and also not that good a passer. Cuadrado is creative, but if he's the plan A, B and C, it's not that hard to defend against him. when Kulusevski went to Tottenham all freedom was taken away from him and he contributes a lot more.