I posted on the Discussion pages of the two projects, but I was said it was better to directly post in the bugtracker.
So when I use UserMerge and Flow extension, I get the following message when deleting a user :
[442729fa14814bb95ac91954] 2016-08-09 15:05:01: Fatal error type « Flow\Exception\FlowException »
To be sure, I deactivated Flow, and i managed to delete my user.
I would be glad to help with development, but I can't ...