We wanted to keep the LoadEntity simple and not pluginify field types so we moved the per field type processing into methods on the LoadEntity. That works OK for core but after writing a sample module for contrib (https://www.drupal.org/sandbox/benjy/2422307) it was obvious the DX isn't really good enough.
Right now as a contrib module providing a custom field type you have to hook yourself into 4 migrations to make sure everything is migrated appropriately. Any less and things blow up.
Add a new plugin type and call that from LoadEntity for per CCK field type processing.
User interface changes
New plugin type for adding support for custom field types in migrate.
PASSED: [[SimpleTest]]: [PHP 5.4 MySQL] 89,261 pass(es). View
PASSED: [[SimpleTest]]: [PHP 5.4 MySQL] 88,517 pass(es). View