I found this very interesting study showing that NADH increases ATP output in isolated cardiomyocytes under simulated stress, whereas equimolar concentration of NAD+ and Nicotinamide don't:
Source: https://bpspubs.onlinelibrary.wiley.com/doi/full/10.1038/sj.bjp.0705300
Source: https://bpspubs.onlinelibrary.wiley.com/doi/full/10.1038/sj.bjp.0705300