archai are taken for the extra protection from mortal wounds, something they'll still have over vargheists, but is mostly only mechanically preferred in the first cohort formation, in which vargheists are not an option.
harbingers are taken for the extra charge distance, which they still have over vargheists and which still makes them a better speedy offensive hammer in many cases, particularly legion of night outflanking, though that isn't relevant here.
In legion of nagash, vargheists have 10 attacks for 160 points, and have a 1/6 chance of making additional attacks when they kill enemy models, compared to morghasts with swords, which are 220 points for 12 attacks (morghasts have +1 attack in legion of nagash). To have the same number of attacks, vargheists have to kill an average of 12 models with their initial round of attacks, which is possible but unlikely. But they do cost less to start. They have the same number of wounds. vargheists have a worse save, and don't have archai's extra protection from mortal wounds, but again are cheaper. vargheists move faster, but don't have the extended charge range of harbingers.
In general, vargheists compare generally favorably to sword-armed morghasts, especially in legion of blood or sacrament armies, but that's not really the comparison you should be making, because morghasts in general should probably be taking the halberds regardless, and then the quality of the attacks generated start looking very different. -2 rend is very hard to come by in a legion of nagash army, and the total damage output of 8 attacks at 3 damage is noticeably higher than that of 10 2 damage attacks even in cases where the extra rend isn't relevant.
In general, I would lean towards morghasts in LoN armies as a result. Again, especially for the extra rend. enemies where that extra rend isn't relevant can probably be poked to death with skittle spears anyway. But vargheists are far from a bad unit. and the comparison is no where near as lopsided as it is for, say, skeletons vs. grave guard.