After Immo off VAG ECU, car will start without a valid transponder (always use a key with transponder even if it is blank).
By using a donor ECU or dash the car also will star, but you will have SAFE in dash due non matching dash cluster data.
Always be sure new (donor) unit serial number and HW versions are matching with original unit. In case of replacements, note that ECU from automatic transmission car doesn't work in manual and vise versa.
To get rid of SAFE, PIN/CS/MAC must transferred from original unit to new (donor) unit. There are expensive tools required to do that correct.
What you also can try to get rid of some ECU errors is:
1. Disconnect both accu (battery) cables - and + for few minutes
2. Use a key with transponder and leave ignition ON for 20 min.
if that one doesn't work
1. Disconnect both accu (battery) cables - and + for few minutes
2. Use a key with valid transponder and turn key 5 times to "ON" and "OFF"