I am not sure it is so powerful. It seems it is only a big deal if you are fighting a lot of lesser enemies, take a bunch of damage up front then manage to kill them over several rounds yourself/with your summons. It's only once per turn & that class feature it terrible if it does not work like this. Still arguably a bit weird but magic.
Controversy noted. You might be right, but consider the following:
1.) Skeletons are already really good at killing things, and they scale up. It's not exactly improbable than any given killing blow might be struck by a skeleton or a conjured elemental. You could wind up with a necromancer who's regaining 15 HP a round (if he refreshes his skeletons using Animate Dead V) during any large-scale combat, with no concentration cost. That's enough to make necro-tanking attractive.
2.) Grim Harvest works whenever you kill a non-undead/construct creature. It doesn't have to be a creature which is capable of challenging you. You could have your skeletons kill 8 wolves (from Conjure Animals) to regain 120 HP. Or just kill 8 chickens, costing a grand total of 8 cp and
zero spell points. It's even thematic for necromancy to ritually sacrifice things in order to suck out their life.
3.) Grim Harvest is already non-worthless due to point #2. You can use it to heal via Vampiric Touch, either in combat (5d6/2 per hit plus 15 HP per kill) or out of combat, for 90 to 150 HP for 5-7 SP.
4.) If you spend a spell slot to create a wight with Create Undead IX, and order it to hang back with a cageful of chickens and sacrifice a chicken whenever you're injured, you'd regain 27 HP per round. And you could arguably keep this up indefinitely with no spell slot cost by just dominating the wight with Command Undead instead of re-casting Create Undead every day.
5.) Even if you didn't have chickens, #4 would let you self-heal 216 HP for 6 SP (36 HP per SP, 50% better than Greatberry) by conjuring mephits with Conjure Minor Elemental. And if you don't need all 216 HP at once, you can use them as scouts in the meantime.
6.) Imagine the multi-classing combos. Grim Harvest wouldn't just work for pure necromancers--it would also be awesome for tank-optimized characters like barbarian/necromancers and fighter/necromancers. Even oathbreaker paladin/necromancers, as if those guys aren't scary enough already.
7.) Since Grim Harvest functions once per turn, and skeletons take their own turns, the "once per turn" limitation effectively goes away. You could potentially regain 27 HP on eight of the twenty turns in a round, for 208 HP/round regeneration. It doesn't even require you to be conscious, so your skeletal minions can bring you back from death's door!
I found Crawford's take on Goodberry surprising but I'd be even more surprised if he thought Grim Harvest were supposed to work this way with Animate Dead/Conjure Elemental. It would instantly go from mostly-flavorful-sometimes-useful ability to brokenly-good must-have-for-all-supervillains. And I'm only 50% joking about the latter.
