Yes, Aubameyang is a good striker, he already proved it in Dortmund, in Arsenal, and in Barcelona.
Although he played for a short time in Barcelona, he can show us that he is a skilled and talented striker. He scored some goals in important matches and become a player who contributed very well. When he was in Arsenal, he becomes the club top scorer, too. No doubt that he is a great striker. Now, he only needs more time to show his contribution in Chelsea, he also needs another striker who can help him to optimize his potential. Sure, Nkunku seems fit with Aubameyang and Chelsea squad.
About Aubameyang leaving Barcelona, I don't think it is a mistake made by Xavi. Xavi knows that they have Lewandowski, it is not necessary to have Aubameyang too. Aubameyang must be unhappy sitting on the bench, he needs time to play regularly. So, to minimize the conflict, Barcelona prefers to sell Aubamayang.
Chelsea reportedly gave a very large value to buy Aubameyang and I think it was a stupid thing for Chelsea to do, Aubameyang has been on the bench for too long, that's the reason I doubt his potential in the future, especially since Chelsea is currently catching up other clubs in the EPL
the transfer market in January will be very busy and Aubameyang's transfer will be one of the most expensive later