Now a issue can be assigned to every user that is member of the project.

It's possible, however, that if a workflow is associated with that issue, the issue with a particoular status can be assigned to a person whose role can't make the issue proceed on the flow.
The issue then become blocked unless assigned to a new member with the "right rights"

I have a suggestion: when a user updates an issue and select the new status, the drop down "assigned to" changes showing only the members that have permission, for that new status, to make the issue change status again