CHAR is not supported by SAP HANA anymore (actually since quite a while) so it's just consequential to not allow it in CDS.
Sure enough there are implications by using different data types:
- char is not stored more efficiently but instead needs to be padded with blanks at the end
- during processing, char requires always the maximum memory, whereas (n)varchar can optimize memory usage based on the actual data
All in all, you'll be much better off using (n)varchar over char.