After posting the question about Harm to heal the summoned undead, I started digging around for spells that could do the same (area negative damage) with fewer actions.
I found none explicitly healing undead, at least Grim Tendrils, Sudden Blight and Enervation all talk only about "living creatures".
Based on the wording of Harm explicitly healing undead, I guess in absence of explicit healing these spells do nothing to undead.
So, does every spell dealing negative damage will just heal undead, or does the spell have to explicitly state this?