Sadly I'm not sure even that's true.
As far as I'm aware xG has a problem comparing players who play in possession based sides with those who counter attack.
Some xG calcs take into account the number of defenders in the area (although I don't think they yet map exact positions), and others try and add multipliers to account for fast breaks, but I don't believe any gets close to being perfect.
This is important for City, as we tend to play against teams that will not only have defenders packed in the box, but they will be about as solid as a defence can be. They'll spend long periods in their exact positions, knowing where there team mates are, and as a result are hard to break down (hence why so much of City's probing play is trying to draw people out of position). Now consider how teams defend against counter attacks - how many times do we see 5 or 6 defenders back in the box but still an easy goal? It's all about the players being off balance, not in a set position, facing the wrong way etc. (Liverpool for example have been exceeding their xG by miles under Klopp, because, while they do have a lot of possession, they are more open, and very good at creating chaotic situations - exactly the kind that xG struggles with).
So, the same shot from a City player - say on the penalty spot - is often likely to much more difficult than it would be for a typical Spurs or Dortmund player (not sure why I chose those teams). The more complex xG calculators will try and compensate, but they can't get it perfect.
So, while it might be useful to compare xG between players in the same team, it's very difficult to compare between teams, especially if they have different styles.
I use both understat and fbref for xG.
Both are great in their own way BUT fbref is IMO the most robust. As you are many of the stats on fbref are powered by statsbomb and they are the fastest growing provider selling data analysis to professional clubs.
Here is the description of how they calculate xG
"Very simply, xG (or expected goals) is the probability that a shot will result in a goal based on the characteristics of that shot and the events leading up to it. Some of these characteristics/variables include:
- Location of shooter: How far was it from the goal and at what angle on the pitch?
- Body part: Was it a header or off the shooter's foot?
- Type of pass: Was it from a through ball, cross, set piece, etc?
- Type of attack: Was it from an established possession? Was it off a rebound? Did the defense have time to get in position? Did it follow a dribble?
There are a number of xG models that use similar techniques and variables, which attempt to reach the same conclusion. The model that FBref uses is provided by StatsBomb. What sets StatsBomb's xG model apart from others is their use of freeze frames. A freeze frame is the location of all players on the pitch at the moment the shot was taken. Was the goalkeeper in position? Was it an open goal or were there a number of defenders between the shooter and the goal? Was the shooter being pressured? Was it a 1v1 situation with the keeper?"
The full article is here:
Based on over performance of xG it is easy to argue that Son and Kane are better finishers than Aguero/Ronaldo because over a reasonable time-frame (understat is best for this to see quickly for the last 6 years or so). In non penalty xG terms Ronaldo has been marginally below each of the last 5 seasons and Kun is marginally ahead. Meanwhile Son and Kane have some big positive numbers.
How to explain this? Kun is the best all-time Premier League striker because consistently he gets off more shots per game on average in the danger areas than anyone else in the PLover the last 10 years via his awareness of space, his strength, his short back lift etc etc. His conversion rate is only just above average BUT thorugh his high shot count he scores more goals. Pure and simple it is a numbers game. With the exception of 12/13 when he was poor, his shot p90 has been in the range of 4.22 to 5.24. His conversion rate per shot has averaged 18.13% with just 3 seasons at 20%+ or 1 goal p 5 shots.
Ronaldo? Average 2+ goals p90 more than Kun. So more goals!