Because of the way Zacks sacrifice ability is structured, you can technically use it during combat, meaning you can block an attack and trigger it to cancel out the attack altogether
But that stuff was pretty huge then
With the high starting life total and the inclusion of Commander damage, life gain strategies are not generally considered very powerful in the format
But other stickers that happen to be placed over the art do not modify the art
If the target creature is an illegal target as Harvestrite Host's ability tries to resolve, it won't resolve and none of its effects will happen
For example, if you side in Pithing Needle against The Aristocrats to deal with Sorin, Lord of Innistrad, you can still cast it to turn off their Falkenrath Aristocrat or Cartel Aristocrat in a pinch