Currently, transfer UI can't edit the config before to preview.
Therefore if the configuration which is created by DataConnector's guess function isn't suitable for the data, we can't import the data and need to use CLI instead of Console UI.
This case happens easily.(for example, when column type in configuration is "long", but the data include empty string)
Solving the case is time-consuming.
When it happens, we need to explain to the client about CLI as an additional explanation. And we ask the client to run the job with CLI.
Addition to it, we often prepare the config file for the client because the client doesn't have enough knowledge of the embulk configuration file format.