I tested this last week. I did the standard built-in "import" from Trello.
It worked great. But we had more business process work to do on the board, so I deleted the Jira board and planned a change freeze for today to complete the migration.
Today, when I do the migration, it brings in the board and the board settings, but doesn't bring in ANY of the 500+ cards. Just no cards - just a blank board.
I got no errors. It's failed 3 times now, so now I have 3 empty copies of the same board imported from Trello.
What did I miss? Why did it not import cards, but instead only imported the greater board structure and left it empty?
I'm not site admin so I can't go into the trash bin and pull that old import out of it. But maybe that's part of the issue - that the items are somehow duplicated in the trash bin?