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?
Hi, I’ll add that with a plastic lined nipple, you’re putting distance between different metals. This greatly reduces their affect on each other, which is always a good thing. The only thing to be aware of with lined nipples is how the plastic is formed at the ends of the nipple. It should wrap over the end, to seal the steel pipe from water. This way everything lasts a long time. Lastly, if you are using flexible connectors to get to the plumbing, they have rubber washers that shrink a bit with heat. So, after a month or so, re-tighten the nuts on the ends of the flex lines. You’ll probably find them to be a bit loose.