I could be really unfair when I say this but...
...Havertz, Pulisic, Werner strike me as the same category of player -- all broke through the ranks in Bundesliga as pretty damned good talents. All wanted by Liverpool, pursued for years, but finally opting for Chelsea, and then somehow regressing.
I'm not blaming Chelsea at all, they all came under various managers, but I agree they made a wrong move, too early, but I can't help but wonder if they'd have thrived under Liverpool/Klopp instead. Klopp has a habit of turning weak players into strong stuff. Fabinho was a Monaco lightweight, Robbo even when he first came in, so was TAA actually but they got to the physical levels required of EPL after first seasons. Firmino even to an extent (also a Bundesliga import), Salah. They all really built up strength even from mid-20s. We did fail with Keita, he remained lightweight to the end but his injuries prevented him from developing really.
Partially, it surely comes down to the club you choose. Many great prospects chose badly and ruined their careers in the process.
However - for Havertz I thought that he'd fit anywhere. He's tall, technically gifted, has that German stamina and mentality, plays well with both feet, has great game awareness and intelligence.
Werner is a headless chicken, always was. Pulisic (and Sancho) never looked like generational talents. I have to say that Havertz did fool me though - and I'm still surprised that he didn't quite make it yet.