"Incorrect string value" while migrating from MSSQL to MySQL with Workbench
I had to do the following:

In the source configuration (ODBC FreeTDS), tab "Advanced", I set:

TDS_VERSION=7.2;ClientCharset=UTF-8


and check "Driver sends Unicode data as UTF-8".
(for more info in http://www.freetds.org/userguide/odbcconnattr.htm )

Then, in the target configuration, tab "Advanced", in textbox "Others" I added the following new line: (change it as you whish, always utf8* )

preInit=SET default_storage_engine=MYISAM,character_set_connection=utf8mb4,collation_connection=utf8mb4_spanish_ci,collation_server=utf8mb4_spanish_ci,character_set_server=utf8mb4


(for more info see:
https://dev.mysql.com/doc/refman/5.7/en/server-options.html
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html
https://dev.mysql.com/doc/connector-cpp/en/connector-cpp-connect-options.html )

Finally in the Data Copy step, in Options group check "Driver sends data already encoded as UTF-8".