Old Dark Harvest was really good/bad based on the number of stacks you could manage to get from actively farming, it was essentially a reward for good laning/jungling on a basic level and on certain patches it was really strong on certain champions and often would give a busted amount of bonus damage over electrocute, though you had to spend time earning it and games could end before this rune could turn on or you could be zoned from souls.
New Dark Harvest cuts the soul game for most of your stacks, which is better for ranged champions, but most importantly 50% hp is easy enough to proc. Any poke champion can do this, it heavily rewards lane and nerfs jungle. This rune is only viable on a jungler if they're an invader like Lee or something unless they overtune it like Riot did. Its a rune thats not going to be viable on everyone, but its doing effectively the same thing in teamfights where most people are low.
Personally 50% is the thing I take foul with rather than the amount of damage you're allowed to do as an execute. Electrocute does this but in a much more powerful combo that turns on much earlier. 50% is incredibly easy to get. Dark Harvest has always seemed like more of an assassin/fighter style rune to me, best on junglers but you could take it in lane even with some ranged champs like MF who follow a similar style. Grab the stack, poke out some damage, buildup your power or go for multiple bursts in a teamfight. With the new condition the pattern is poke, poke, poke, get easy stack. Poke, stack. Poke, stack. Poke, stack. Its useful for fighter/assassins too, but I think this thing is way too easy to get as is. If it was 30-40% assassins and fighters could activate it, but it'd take more work for an adc/mage to use it but as is its stupidly easy to farm.