The thing is Serie A was close to the level of the PL in the 00s and Shevchenko came here. He just had too many minute s in his legs to adapt and lost a yard or two.
Nowadays every Euro league is miles behind the PL so every signing is a gamble. You just have to see the right qualities in a player that could help them adapt quickly but the mental challenge of dealing with the relentless grind and a constant slew of tough opponents every week is unknowable until they get here.
I think the generally accepted issue is that Serie A is slower, and gives forwards too much ball time. They then suffer to match their ability to the chances they'd get in this league.
Similarly, La LIga and Ligue 1 have a real drop in quality towards the bottom of the table, meaning you get a lot of stat-padding. The PL is fast and unrelenting, so it's common to see a washed-up PL player being prolific in Serie A (see: Giroud) but there are barely any modern examples of Serie A forwards coming to the PL and finding their scoring form. That for me is one of the red flags with the Moise Kean and Lookman types that rejuvinated in Serie A