I have thought about this a lot and while I think Immortals have a place, I don’t think people recognize just how many shortcomings immortals have.
The biggest one their damage against light targets. Everyone knows that Immortals have great DPS vs armored but if you do the math for their dps vs light, they are one of the lowest DPS cost per units in the game.
Don’t believe me? Two stalkers do more dps to light targets than 1 immortal. And the 2 stalkers actually cost a little less than an immortal to boot. Plus the stalkers are much faster and can get blink. The immortal is a clunky liability in many situations and needs, at minimum, force fields to not get jumped on. And usually needs a prism as well.
Now if you have an immortal with a prism the math gets even more fuzzy. Now you are paying, on average 5 supply, 400 minerals and 100 gas for each immortal if you get two immortal and a warp prism. For 375 minerals, 150 gas and 6 supply you can get 3 blink stalkers that do over 50% MORE damage to light targets than the immortal. But you probably think that two immortals in a prism blow 6 stalkers out of the water in DPS to armored targets right? Actually its only 20% more dps.
Now there are a few things that you could argue in favor of the immortal here.
- Obviously with the example above the immortal is still much more supply and cost efficient versus armored targets
- Immortals have shield.
- Immortals are a single strong unit which has a lot of advantages. E.g. If it lives longer then it outputs more damage than blink stalkers that get focused down one at a time. Plus better against AOE. Its the same reason you cant look JUST at cost per DPS for things like Stalkers vs. Marines.
But the worst thing about the immortal is that it costs valuable robotics facility production time. So its laughably low dps/cost to unarmored targets, complete reliance on a warp prism or at least force fields to keep it alive, and it being produced from the robo facility make it have a pretty short shelf life.
What do I mean by short shelf life? Well, it just doesn’t compete in the late game. Immortals are bad vs bio in TvP generally. Splash damage is much more effective vs Terran and Zerg, especially late game which comes from the other robo units. Immortals are a great fighting unit to get if you know your opponent is going roach heavy or (not that this happens too much) Marauder heavy. But its shelf life is limited because 1. If you over build immortals you wont have splash from your robo and worse, your opponent can easily build a comp to stomp immortals (e.g. morphing roaches into ravager and swapping to ling bane rav). Theory crafting with Immortals doesn’t work past the mid game. Yeah they beat thors… but are going to get stomped by the rest of the supporting army because of their mobility. Decent against tanks in the mid game in low numbers. Against zerg, any decent Zerg completely switches out of roaches after their first max out, if not before. And then almost the entire tech tree other than roaches hard counter immortals.
So what is better than immortals? Well I already think immortals are not too great against Terran usually. They are slow and clunky, get owned by marines, and even if you get a warp prism, the Terran has a lot of tools to shoot your WP down. Against Zerg they are standard to be built versus roaches. Good with shield WP or shield batteries. But still very very vulnerable if left alone or without a warp prism. So I think Archons are better in many cases. They have bonus damage vs the units you would want to use immortals against (roaches). They are actually almost as fast as Stalkers. They are very tanky. Dont cost much total resources than an immortal (but are gas heavy). Can be build from Templar so you can get use out of the templar first and do not use robo production. Plus immortals do splash, plus they are good against unarmored units too.
TL;DR: Immortals are a limited mid game unit. And IMO get outshined by Archons as WP juggled or even front line units in PvZ in many situations. In PvT both are not as useful.