If a particularly shooty vehicle (generally among my first targets) chooses to pop smoke to try and stave of death by lots of melta, instead of getting off a last hurrah, I'm remarkably fine with that. Especially as it still tends to die.
Or that, yes.
If I'm playing against Eldar, I know one thing for sure: They move 12" and shoot with all the weapons they need to shoot with every time all the time and always have no less than a 5+ cover save regardless of terrain. If they're jetbikes, they also move 2d6" in the assault phase so they can shoot and then get behind a wall and lol no shooting back for you.
Also they have an effective range of 24"+, my effective range is 12-24", my meltas have 12" range and the models they're on move 6" per turn versus their 12".
Ignoring their cover save is the last of my concerns.
Even against other armies, popping smoke to not die against 4 meltas is probably the stupidest thing my opponent can do. If I'm in melta range, it means I can shoot 4 meltas and, in the unlikely case the vehicle survives, assault and krak and melta bomb his butt, which also nets me a few inches of very precious movement.
Long story short: it's a friggin' 12" weapon, if I'm close enough to use it against a vehicle, it's because he's either stupid or has nowhere to run, and at that point I don't care about his smoke launchers or the fact the entire game table is area terrain. If I'm close enough to be in melta range, I don't care even if he has 2+ cover.
And about storm bolters: No. At 5 points per model, no. "But!", you say, "You can
once per game shoot at 24" away ignoring cover!", yes, and kill what? Imperial guard, kroots, cultists, orks? And to be really effective, I'd need a full squad of 10 with 4 storm bolters, and that costs me about as much as 4 meltas in an outflanking immolator with twin-linked multimeltas and doesn't help me one bit against vehicles, monstrous creatures, terminators and other nasties.
If you're that concerned about gunlines hiding behind walls and fortifications, why aren't you playing more Seraphim? They're still very good, and their hand flamers got halved in cost, and if you stick a melta bomb on their superior they're even decent tank hunters thanks to the fact they move twice as much as a dominion and jump over terrain.
No, I'm sorry, "Ignores cover" when your options are either meltas or AP5 weapons, one of which
already ignores cover, is definitely a nerf when the previous option was "Twin-linked", at least because it didn't completely invalidate one out of three weapon options.