This limitation is implemented for Oracle database only. Some Oracle versions have limitations in using long custom field names on some scenarios that lead to import errors.
However, I will add this as a feature request for additional checks in eazyBI advanced settings or support the long field names for newer Oracle database versions.
For the workaround, you may rename the custom filed (add a shorter version of its name) in eazyBI advanced settings. The settings might look like this where NNNNN is a custom field id in Jira:
[jira.customfield_NNNNN]
name = "Shorter version for cutomfield name"