But that's the key right there isn't it? Take a look at all the other "powerful" abilities currently in game.
1) Shield Regeneration. Replenishes shields. Powerful because it is quite good at keeping nearby advent ships from exploding. Why is that powerful? Because all the credits/metal/crystal and time you spent building the rest of your fleet becomes more effective and can deal more damage than it would without (since they might be dying). On it's own, Shield Regeneration isn't even a threat to your enemy.
2) Malice (pre-nerf or not). Powerful because it essentially multiplies the damage you're doing with your other ships and inflicts it on more of your enemies fleet. Once again, on it's own it does very little and isn't a threat. The "powerful" nature of it is dependant upon all the credits/metal/crystal and time you spent on the rest of your fleet.
3) Flak Burst. Helps eliminate hostile strikecraft. Powerful because strikecraft can be very dangerous, moreso because they cost nothing but antimatter to build once you pay for the carrier. Does no actual damage what so ever to the enemy frigates, etc that he had to spend resources on. Helps you win because it tips the balance and lets the rest of your fleet that you spent credits/metal/crystal live longer and be more effective.
4) Nano-Dissasember. Does damage directly to hull and lowers armor for a bit. Does this to a single target and deals about 600 - 1200 damage (30/sec for 20 - 40 seconds) total by itself. That's not enough to destroy much of anything except the lightest of frigates on it's own at level 1 and even at level 3 isn't enough to take out a single heavy cruiser. Armor decreases, once again, are of most benefit when the rest of your fleet that you spend credits/metal/resources on can take advantage of it.
There are more but are we seeing a pattern here? The power of most of these "best in game" is extremely reliant on the rest of your fleet. They scale with fleetsize but only if you've actually improved your fleet along with them.
Now, some good abilities that don't work like that so much.
1) Resource Drain. Does a decent amount of damage to a single planet and steals some resources. The numbers are 45 health/sec and 2 pop/sec for 30 seconds while you gain 15 "resources"/sec. It hurts but it's not enough to take out any upgraded planet other than an asteroid by itself.
2) Phase Missile Swarm. Does up to 600 "shared" damage to up to 7 targets.
3) Gauss Blast. Does up to 800 damage to a single target.
4) Cleansing Brilliance. Does 250 "shared" damage per second for 8 seconds to as many ships as fit into a 8000m x 1000m radius column. This is the ability that most closely resembles Missile Barrage but does a maximum of 2000 damage to a target and the column radius largely limits it's total damage potential. Can be interrupted.
As we can see, aside from Cleansing Brilliance, the better abilities that aren't reliant upon the rest of your fleet for their power are tightly constrained in just how much they can actually do. Cleansing Brilliance, on the other hand is not generally regarded as being unbalanced because the column nature of it's effect generally keeps it's overall total damage in check. Generally speaking, I figure it's rare that you mange to hit more than 20 - 30 ships at best with it in a single blast for a total damage of 40,000 - 60,000. It also means that the Radiance has to do do some maneuvering in order to get a "good" shot off.
Now let's look at Missile Barrage.
150 damage a target 20 times to as many targets as will fit in a 10,000m radius circle. The math works out to 3,000 damage to each target, or at least some portion of that even if they manage to clear the radius before it's over. That doesn't seem too bad except that 10,000m radius circle is HUGE and covers a sizeable portion of a gravity well and most likely the majority of the space where a fleet battle is taking place. That effectively nullifies most of the limitting factor on it's damage potential as it could conceivably hit the entire enemy fleet of 100+ ships. No special maneuvering is required as it's a circular area of effect, you just have to get close enough. 3,000 damage also happens to be enough to destroy many of the various types of frigates and cruisers in the game.
So we can see that the basic problem with Missile Barrage compared to most other abilities is that A) It's unreliant upon anything else for it's effectiveness and B ) That it's "limitting" factor doesn't really do much to limit things and keep it in check.
Solutions? A couple quick ones.
1) Reduce the circular radius.
2) Cap the total number of targets that it can hit per volley. (12, 18, 24, whatever).
3) Change the area of effect. Perhaps to a column like Cleansing Brilliance but it could be wider (2000m, 3000m, 4000m, whatever).
Any of these three or some kind of combination them would effectively keep Missile Barrage in check in terms of it's total damage potential and still leave it more than capable of inflicting a ton of damage. I could mod any of these 3 solutions in under 5 minutes if they wanted to be tested.
Another solution, as others have suggested they would prefer, would be to increase the effectiveness of other abilities to better match Missile Barrage. That would work too but is a far larger change to make as instead of balancing one ability/ship you are instead rebalancing the entirety of the rest of the game. Which do you think is more likely to be done by Ironclad? It also doesn't just affect capital ship balance, it would change the balance between capital ships and frigates as well - probably to a large degree.