Mortal wounds are awesome, but we have to pay waaaaay more points than most other armies to get them. I think you'll find that at least with the Skyrays, from an efficiency analysis, you are just as likely to get a wound from larger volume of fire through purchasing high shot output models than from the mortal wounds from the seeker missile (especially factored over multiple rounds). Also, Skyrays are darn near impotent after their seekers are gone when considering their points cost. On average you will get 13 hits with 5 markerlights which isn't even enough to fell a Landraider. If you do get lucky enough to get all of them through, congrats....you just used 600 points to kill 300 points. This is also taking into account that you gained first turn and the enemy has not had a chance to either kill your skyrays or kill your markerlights...
Outside of the Skyray and especially if you have already planned to invest in a sure means of 5 markerlight hits on a target then the efficiency model shifts a bit when you are talking about additional mortal wounds (rails) or cheap add on single shot mortal wounds (seekers and destroyers). BUT....the units that can take seekers or that deal additional wounds are already heavily overpriced to begin with for the most part.
Rail Rifles - extremely overpriced for a BS4+ S6 (wounding most vehicles on 5+) weapon when compared to similar weapons in BS3+ armies
HRR Broadsides - no explanation needed here...I think we all know they are priced out of usefulness
Hammerheads - looks good on paper, but it's a lot of points for something that only does mediocre damage on its own and rarely ever gets off a mortal wound (can take seekers for a bit more firepower for minimal cost increase...not bad with Longstrike support, but then you'll need to build your army around protecting your hammerheads)
Devilfish and Piranhas suffer the same points overcosting per usefulness dilemma on their own, which makes them a bit toothless after they have let go of their seekers.
Best sources of Mortal Wounds
Longstrike - good points efficiency, good buffs, good source of marginally reliable mortal wounds
Sunshark - high mortal wound output against certain targets
Stormsurge - High damage output already and still extremely potent once its destroyers are gone
To secure victory, the wise must adapt ~ Puretide