Most of the basic field types and some of the others will work as expected it you need to rebuild a field group.
The problems come in for more complicated fields.
For every field acf stores two values in the database, for example lets say is a simple field.
The first entry in the database is your field name with its value
meta_key => field_name
meta_value => your value
the second entry is the field key reference
meta_key => _field_name
meta_value => acf field key example: field_1234567890
for many fields, acf doesn’t know what to do with the value unless it can find the field key reference and this is why the more complicated fields end up blank.
