Time limits seem fine to me.
Out of the three, my only real problem is the Vulture, but only in terms of the fact that it takes SO long for it to actually start shooting at people because it has to take seconds to get its laser sight on someone before it does, so if a person appears in front of me, I'll usually kill them long before the laser actually gets on target, and if someone gets behind me, I'm usually dead long before they can be targeted and shot, which makes the "watches your six" description pointless.
You'd think it'd be like the Sentry Gun where it pretty much just auto aims instantly and starts shooting as soon as an enemy appears in its line of sight.
I agree with your point as well they need to make it more useful. But the time limits are too short if they going to be that short for something so easy to destroy then the score streaks need to be easier to get as well.
I think the kill streaks are fine - the only one that might be a little OP is the Helo Pilot - seems to go on forever.
I do try to take them down but even with Blindeye they seem to find me.
And yet you have people who think it should come with rockets/missiles, lol. I do admit that the flares come off as useless given there's no actual lock-on Launchers in the game that would warrant their use. The closest would be the MAAWS, but that's manually guided using a laser, not lock on fire-and-forget missiles.
I'd say it's not near OP simply due to the fact that it's a 12 point streak and, unlike BO2 (where the Scorestreak system was too rewarding, I think), you only build up through kills and destroying opposing streaks, so it's not too easy or quick to get, coupled with the fact that it only has the cannons as a weapon, requiring you to actually move around and angle right (though, it seems to compensate with a deceptively large hit radius). If not shot down, then the best tactics would be to simply hide deeper inside enclosed buildings (away from doors and windows, of course, and Blind Eye also helps), or to stay as far underneath it as you can.