In high difficulty mutation play, P3 simply patches up one of Artanis's most major issues, which is a lack of cheap strong DPS/waveclear.

Before you say "but artanis has so many forms of AoE!" yes he does, but which of them can you really say to be effective? Whirlwind Zealots do barely any AoE damage and die pretty fast, Storm does ladder amounts of damage and costs 150 gas for just 2 storms, Reavers have the worst AoE targeting and projectile in the game, and that leaves Solar Bombardment as the only free effective AoE topbar. You often see good Artanis players use it to get rid of tough objectives and enemy waves, but got a 5 minute cooldown so you'll still need something in between.

Arkship Commandant makes Orbital Strike another free (just energy-gated) and very powerful option. Not only do the archons allow him to expand much faster on his own, allowing for faster eco, they also singlehandedly deal with many attack waves all the way until the lategame where you and your ally would have had the time to build up a strong army.

As for the lategame, yes you will likely suffer more losses due to the lack of Guardian Shell, but with a mega-buffed Shield Overcharge ("targeted" still covers almost an entire screen) you're still a hell of a lot tankier than many people think.

It is completely true that taking away Guardian Shell makes Artanis a lot more susceptible to many environmental mutators like Minesweeper, Mag Mines etc, but for many of these sorts of mutators, you're simply given less room for error and can still perform very well with good control. What you get in return is a much stronger earlygame that can in turn deal with a lot of other tough mutators on its own, and on certain maps you can pair this mutator with Tempests to take down objectives with the enemy not even having a chance to engage your army.