Hi
You are correct in saying that S4 HANA has completly revamped the architecture of the back end tables. However, the existing reports will work as well, only the data will be fetched from tables like ACDOCA and MATDOC, which are the underlying line item tables. Now the old reports were looking at tables like MSEG, MKPF, BSIS, FAGL* to name a few. All of these have been converted into views and when you do the migration from ECC to S4, the underlying code for these reports also gets updated.
In addition you have line item reports pointing to hana, for eg, for GL line item, the additional report is FBL3H. So you can check in table TSTC for t codes ending with *H.
Additionally the reason i said Firoi, because there are lot of standrad applications, where the data available and the user experience is much more superior to standard reports. You have to check on your business requirements and check which applications are suitable for you.
In addition to Fiori, there are standard HANA Live views which greatly enhance the reporting and KPI capabilities in S4.
Regards
Sanil Bhandari