The Hunter-seeker is just an inefficient spell. getting it when you are not in a huge lead can put you behind. And its design is bad: it's very similar to a scarab from SC:BW, but thrown from the air (flying scarabs! imba right ), but it doesn't cost minerals to build, it requires a ton of energy and is easily countered and dodged, unless you want to get close and sacrifice a viking to get it in.
1)The raven costs a lot of gas, gas that a Terran needs desperately, you use gas for tanks, vikings, banshees... pretty much every unit excluding marines and hellions, so the only situation i get ravens out is if i need mobile detection, or in some cases for the point defense drone (which is also effective not efficient, but since i already have a raven out, might as well use it, but that's another story)
2)The Hunter-seeker needs to be researched which adds more gas cost, and time to research making it a late game spell
3)The hunter seeker does 100 AoE damage instant, while some would argue that this the best of its equivalents, it simply is not the case. Energy cost wise 2.5 fungal growths deal the same damage, storms would over time deal more damage as well. Hunter seeker also damages your own units, so the only good units to use it against in a scirmish would be on the clumped enemy ranged units (i.e. Hydras), which brings us to the next problem, the cast range.
4)Cast range and dodge ability. The cast range is relative, if you want to make sure it hits you need to get closer, getting closer means the raven is likely to die, casting it from far makes the enemy more likely to dodge it, since it moves extremely slow.
5)125 energy cost! It needs to be researched.
All these points add up to an unusable spell in terms of efficiency.
1)200 gas could be used to get 2 tanks which will do more damage than 1 hunter-seeker, a raven can't cast more than 1 in the same battle.
2)200 gas could be used to get a ghost which has EMP, a better spell for the same amount of gas + no research cost.
3)In every match up, other units are more rewarding for the same gas investment.
In TvZ its better to get tanks, they will kill more units than ravens with the same gas cost.
In TvP massing ravens is not an option since you are saving up your mana which BEGS you to get Fed-back by templars. if you have a raven out better to use the energy for point-defense drones.
In TvT you just don't get ravens often, TvT is often Viking heavy, and the only use of a raven in that MU is to cast point-defense drone in viking skirmishes.
As mentioned before, get 2 tanks and voila, same damage, or get something else. Is it a good investment if you already have vikings out? No, use point-defense drone.
I don't want to sound regressive but i wish i had Irradiate over this spell.