You keep bringing this up without looking deeper. In his first season at Valencia (18/19) he was mostly coming off the bench as a 18 yo kid in a solid side that finished top 4 in the league. Marcelino played likes of Rodrigo, Santi Mina and Gameiro as strikers while Ferran was more of a RM in his standard 4-4-2 double 6 formation so not that close to the goal.
Next year Marcelino was fired early in the season due to disagreements with Valencia board and was replaced with Albert Celades who is not a good manager. So unsurprisingly they've struggled for most of the season. Their top scorers Parejo (mostly from penalties) and Maxi Gomez have scored 10 goals so hard to stand out in a team that has problems to score. More so, after corona break Ferran's role was diminished when he rejected to renew and Valencia already decided to sell with a year left on his contract.
In his first season at City he scored 13 goals in all competitions, including 4 in the CL. Pretty good stats considering it was his first season in a new league and Pep playing without a clear #9 rotating his midfielders and forwards around a lot so they ended up with 7 players in double digit goals and Gundogan leading the team with 17 total goals. And we clearly can't say much about current season when he has been out injured for the last two months.
I think his numbers will improve if playing regularly. Xavi wants to play similar system to Pep so he should fit in fast, he also knows players like Pedri, Gavi and Eric. OTOH of course there will be bigger pressure on him to score more as we won't have another 5-6 players capable to score 10+ goals like City does.
Don't see him becoming a 30-40 goals scorer but he should get around 20 goals (and depending if we get a top striker alongside him he could also add around 10 assists) every season imo. Also not sure if he should be a false 9 longterm and considering we're linked to likes of Haaland and Vlahović that's probably also not our plan but for second part of this season with lack of better options he is the best fit for that role alongside Ansu if/when he'll return. Ferran's ability to play as a false 9 clearly adds to the team's versatility and unpredictability though and he could also play there if whichever striker we'll bring wouldn't be available.
This is probably one of the reasons why Xavi prefered him over more pure wingers like Raphinha or Antony. Another reason which was already mentioned is his adaptation should be much easier which is even more important when signed in January so he won't have a luxury to have a month of preseason to gel with the team. When fully fit to play I think he can make immediate positive impact but hopefully people won't expect too much from him too soon (who I'm kidding people will have patience though, lol).