It preserves custom MX records. If custom MXs are detected, it also copies across TXT records (for SPF/DKIM).
I've just run the newest cPanel importer, and it's unfortunately no better. Our cPanel dns zones use the unadorned (no host) domain as the MX record, and the MX record which is NOT custom at all is preserved when it should be converted to mail.<domain> because email is hosted on a different server which just adds more work for us rather than less! This is not a custom MX record if the MX points to the same cPanel server hosting the domain!
Now the only way I can see this working is if we can rework all cPanel MX records automatically from <domain> to mail.<domain> for all cPanel zones, but then I'll have to contemplate the ramifications for actual custom MX records.
I sincerely hope Enhance's cPanel importer can resolve this problem quickly!
This is worse, not better.