let's compare the most common artillery vehicles ingame, shall we:
Whirlwind: probably the most effective artillery vehicle. devastating to pretty much any enemy unit that's not a lategame elite. disrupts line infantry and even some entrance elites easily. Solaria's presence makes them even more godlike giving it stronger attack and damage over time. also Thunderhawk doctrine can withdraw them quickly from danger. things that balance this out: long ability cooldown, paper-thin armor, has no weapon outside of Q ability for self-defense
Big Trakk: a very solid tank/artillery hybrid. can switch between fire modes and can fire continuously unlike Whirlwind. also takes quite some punishment due to heavier armor. may not go toe-to-toe with an Annihilator but is still a great fighting vehicle. becomes even more devastating with Squig Spotter and its high firerate, not to mention, it fires automatically with it like a sentry.
Fire Prism: here, we have a skimmer unit which has a really long-range attack.... that's it. damage is not really anything impressive. its delayed explosion is not anything impressive -- not to mention, infantry has long gotten away before being disrupted. takes quite long to charge up as well even if opponents are already within firing range. not to mention, paper-thin armor! doctrines are subpar, i don't think Crippling Blast does anything useful. Warp Spider's teleport presence is quite interesting, but too short-range and too situational. don't get me wrong, the Prism is NOT a terribly useless vehicle! but it tries to be both a main-battle tank ("Eldar's Annihilator") and artillery unit but fails quite hard at those, in my view.
Please tell me if i miss anything. Eldar lacking a proper artillery unit is certainly an important factor especially in team matches with lots of stalemates. but i would not know how to solve this outside of a straight OMG BUFF DAMAGE! ...please feel free to discuss