It is similar. For that, they would be attacking with the xbow first (or they lose the spell). So you have the delay between them shooting, hitting lasttarget, and the half-second damage delay of meteor swarm. With instant hit, the ms damage would come nearly concurrent with the heavy xbow hit. Additionally, when you see the opponents coming in, you currently get the knowledge of the potential danger since they already have their weapons equipped, and they have to stand still for 1 second before they can fire their shot. It is pretty similar, I'll give you that.
Insta-hit is not necessarily good or bad, it's just different and would likely need many other changes to the server to make it work. When you see an opponent equip an item, the current setup gives you the opportunity to prepare for the potential damage. You get the opportunity to counter to defend yourself. Insta-hit doesn't allow for that. Someone can just inject instant melee damage that cannot be prepped for.
As long as people understand what they are getting...it's fine - but I would anticipate many additional changes to compensate for the change.