I haven't noticed anything much wrong with him as a footballer compared to Welbeck!
But it does seem like he's allergic to playing football in England.
By his own admission he had a personality conflict issue of some kind with Wenger and was excluded and disgruntled, and wanted to leave. And Laca was also involved. That already makes it hard for him to ever come back.
Emery arriving as manager opens the door a fraction, but if he's still damaged goods in the eyes of his teammates it's better just to let him go.