Our current forum is running on 2.4.24, so this was my baseline upgrade after making a copy of everything. I ran this upgrade out of the box without checking anything prior.

1) Our DB does have duplicate email addresses and the upgrade correctly flagged the culprits.

2) The one upgrade error I received was the following:
Error!
Database error in line 298: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ' KEY `b8_spam` (`spam`), KEY `B8_training_type` (`training_type`)) ENGINE=Inn...' at line 1

With a quick check of Line 298 in update/update_2.4.19-2.5.php, I see 2 commas after PRIMARY KEY (`eid’):
if (!@mysqli_multi_query($connid, "CREATE TABLE IF NOT EXISTS `" . $db_settings['b8_rating_table'] . "` (`eid` int(11) NOT NULL, `spam` tinyint(1) NOT NULL DEFAULT '0', `training_type` tinyint(1) NOT NULL DEFAULT '0', PRIMARY KEY (`eid`), , KEY `b8_spam` (`spam`), KEY `B8_training_type` (`training_type`)) ENGINE=InnoDB CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;")) $update['errors'][] = 'Database error in line '.__LINE__.': ' . mysqli_error($connid);

3) After changing the code in the script and running it again, the upgrade was successful.

4) I didn't see the same issue in any of the other CREATE TABLE scripts.

5) I will also run through a 2.4.24 --> 20220803.1 --> 20240308.1 upgrade, since I'm guessing many forums are currently running off 20220803.1. Will report back.