So, with PEX-lined steel nipples, the main thing is that there is no dissimilarity of metals, since the PEX lining reacts with nothing. Stainless steel is not the same as the steel a water heater is made of. It’s an alloy and some distance up the galvanic scale from carbon steel. So you could have some electrolysis between the two. That said, it’s not going to be much. Plumbers for decades have used brass nipples, which also are up the scale. With either, it just means the anode has to work a little harder.
Still, why would you spend more money to achieve an inferior result?