Can I use forecast data in ERA5 MARS(stream=oper, type=fc) as NWP forecast?

I am working on a atmosphere-to-power AI model on a solar power station, and I am not sure whether I can use ERA5 MARs forecast data (stream=oper, type=fc) as NWP input data in my model.

From ERA5 documentation, I see there are forecast data from IFS(HRES). However, someone told me the forecast data in ERA5 is not real-time predict data (like HRES and GFS), and thus can’t be used. He suggest me use GFS prediction data instead. He can’t explain the detail reasons.

Could anyone give me any suggestinos? Thanks!