We have performed several IPB 4 migrations so I think I can answer. Last one was Guineapigcages.com which switched from IPB to XF 2.3.
1) The the official XenForo importer is robust but every forum is unique. Since yours dates back to 2018 you may encounter orphan data which are usually...
This is almost certainly caused by missing or corrupted files. Are you uploading the files through FTP?
If so, ensure the FTP client is set to binary mode and configured to overwrite existing files
If you have installed this addon https://xenforo.com/community/resources/digitalpoint-app-for-cloudflare-r.8750/ then yes. Otherwise you have to log in Cloudflare and do it there:
XenForo does not natively support embedding WebM files directly via URL or attachment in posts but Andy's addon supports that file type:
https://xenforo.com/community/resources/video.5701/
Check the values on IPB:
SELECT g_id, g_name, g_order
FROM core_groups;
if any of them is inconsistent (null, non numeric or empty) fix it by running this
UPDATE core_groups
SET g_order = g_id
WHERE g_order IS NULL OR g_order = 0 OR g_order = '';
Also make sure SQL is not on strict mode.
The error is during the import process which means is at the source. That means you should look at IPB, not XenForo (which will display the usegroups successfully migrated)
If it helps, we could set up a testing site on one of our domains and import your forum so you can check if everything is OK before purchasing the license.