Also note that it's not just a spot
that Picard specifies for the attack - it's also a moment
. Several starships are destroyed and hundreds or thousands of people die while Picard tells the fleet to wait for it, wait for it, NOW!...
This fits well with what we know of the Borg and their technology. There are
no weak spots. Why? Because everything is decentralized and rerouted around damage. The target keeps moving - the bridge is here now, there at the next moment. But if you are part of the Collective, like Locutus is, then you can tell how
things are moving. And then you can fire smack in the middle of where a weakness will be at the time of impact.
Sure, they thought they had taken Locutus out of Picard. But that would not be consistent: several other episodes show Federation medicine can't remove even much coarser bits of instrumentation from a brain without killing the patient. The Borg nanogoo in this movie causes large metal objects to pop through the skin eventually. Surely it would also be plausible for the unnoticeably thin goo to hide in Picard's blood for a few years, then suddenly assemble into a tiny subspace transceiver somewhere inside him (and again dissolve after no longer needed).
As for "old" weapons still having an effect, it must be a question of magnitude to some degree. The Borg may adapt so that a Drone can shrug off a hand phaser beam, but it doesn't follow that a Drone could then shrug off twenty starship main battery beams of that exact same sort...
OTOH, we have to consider that the Borg don't wear their adaptations all the time. The first few Drones always succumb even to "old" weapons before the Collective decides it's had enough of this and adapts. It's just that the Collective can afford to lose a couple of Drones in every firefight. It can no doubt also afford to lose a few square meters of hull plating for the same noble purpose of finding out more about the enemy weapons and tactics. Except, that is, when there's a Quisling telling the enemy how to fatally exploit the magnanimously donated square meters.