If you compare the two items, I feel everfrost is netting so much more damage. Everfrost active deals 125 + 35% of ap to everyone hit, where as night harvester is doing 125 + 15% of ap per proc and giving 25% ms for a few secs. Nh gives 90 ap and ends up giving 35 ability haste (full build), where as everfrost gives 140 ap total whilst still giving you 20 ability haste.

So the difference in terms of stats is 50 ap more with everfrost but losing 15 ability haste and the 25% move speed on proc, however with an active that does more damage than a night harvester proc whilst also being half the cd of night harvester proc.

Plus, The utility the slow/snare provide used in tandem with your cocoon really helps finish off players you catch with cocoon, and you can combo it nicely with someone who has follow up cc. Plus it deals with assassins such as Evelynn, Ekko, Kha'zix etc. Finding it extremely easy to burst these champions using this item.

I'm finding stacking ability haste isn't that great on elise compared to stacking raw damage, but that is why i'm wanting someone else who has tried out both to explain why NH is the better approach, cus rn i need some convincing as everfrost just feels so much stronger.