Sure, that would be interesting too. But what it does is that signing from certain leagues may potentially mean more risk. Bundesliga games are often very high tempo and open, so attacking players put up really strong numbers but can struggle to replicate that elsewhere.
But I agree league change isn’t the only factor, Sancho at United for example has dropped off massively, but the fact is the way United play doesn’t really suit his strengths at all. Had he gone back to City I’d fully expect him to be doing well.