Well, I agree on the fact he will no doubt face better opposition in EPL. It's hard to judge a player and perfectly predict how they’ll transition to a new team/league. There's a lot of examples of Bundesliga players that do great in EPL or La Liga, but there are some cases of others who simply can't adapt. Adaptation is part of football, but it’s not always the only judgement a player can have if to catalog him as a good player or not.
He needs to prove himself against top clubs? Sure. So far he did good against Bayern and BVB domestically, the only thing is he was pretty erratic against Bayern in the last game, however he had good matches against Spurs. He also showed good prowess at international level against Benfica, Lyon, Monaco and Napoli.
As I said on Discord, Chelsea doesn't seem like the right club for him. Chelsea is more ball possesion oriented, slow building kind of playstyle (amine's words) and also that was my impression on them on the few matches I watched from Chelsea at UCL, and that's not precisely a style Werner is used to. I'm not saying he won't adapt, but that's not the perfect playstyle take advantage of Werner's assets.
Also, Werner could easily scored like over 40 goals this season, the amount of verticality and quick transitions and space his teammates make for him on Leipzig's system is quite admirable, but he is so erratic and dumb sometimes; on a good day he is capable to score a brace and assist or a hattrick, on a bad day you'll barely notice he is on the pitch. If we check his statistics, yeah sure he has 25 goals just in Bundesliga, but he only in almost half of the matches: he had 14 matches when he didn't score! and usually when he doesn't score he also doesn't make assists.
IMHO Werner does not deserve high SA, because he is capable of wasted a big amount of chance because his shooting accuracy is awful sometimes, like I said above, if maybe another more clinical striker like Lewandowski had the same amount of 1 on 1 or shooting opportunities like Werner, he would have score a lot of them. Sure Werner scored a lot of goals, but he also wasted a big amount of chances.