There are some common challenges during the UCX migration which are published here. You may need to perform a few manual steps if you are migrating from an external Hive Metastore. 

can you specify what are those in short.

Network Issues: Ensure connectivity to Databricks, GitHub, and PyPI. Use VPN or proxy if workspace is isolated.
Proxy/SSL Errors: Set certificate-related environment variables for proxies or self-signed certs.
Admin Privileges: UCX install requires workspace and sometimes cloud/account admin rights.
Version Checks: Use Python 3.10+ and Databricks CLI v0.213+; always run latest UCX version.
Authentication: Proper CLI profiles for workspace and account must be configured.
Multiple Profiles: Select correct profile with --profile during install.
External HMS: Clusters and SQL warehouses must be correctly configured for external Hive Metastore access.
Post-Install: Verify UCX schema and jobs in workspace, run assessments to check migration readiness.

Guess you will find some differences in before and after, such as the use of the wildcard to point to folders in ADLS2 for external tables is supported in hive but not in UC catalogs.