Used this tool yesterday after restoring a backup from a FreePBX 2.8 system onto a new FreePBX 15 install.
The process worked generally as expected.
There were some odd errors about failing to remove some voicemail folders and then also I think some of the default settings did not get set. Because a bulk export of the extensions shows really different between an extension that has subsequently been edited, and ones that have not.
Unfortunately as it was a time crunch due to the fact that the 2.8 system was failed, I did not spend time trobleshooting.
But most importantly, calls were working.