It actually showed how much more powerful the Borg are than the Federation. Picard's orders were to investigate and keep the Borg busy while a fleet was being prepared to properly fight the Borg. Picard went in knowing it could well be a suicide mission or at best, a game of hide and seek, to keep the Borg occupied.
Yes, Picard knew from the start that the borg's military technology is far better than the federation's and that the borg massively outgun his ship.
He also knew that drones can easily teleport through federation shields and are, individually, more than a match for his men.
And he found out that the borg wanted him - personally.
Yet - there were no additional security personnel (or measures that fulfill the minimal standard of springing into action at his capture - we're not even talking about being effective) on the bridge or any other part of the ship; no other measures to prevent his capture (yes, up to and including suicide pills).
As said - Picard was either incompetent or ridiculously naive; most definitely culpable.
No one knew that the Borg wanted Picard, until they met up with the Borg ship. Infact they were surprised about the sudden shift in the Borg's intent.
So the whole idea of adding extra security to the bridge was not even a consideration.
Assimilation was not even know about at that point either.
and
After that it was learned that by being assimilated your knowledge is now a part of the Borg.
[...]
They didn't know about assimilation only that the Borg wanted Picard. They didn't even know why they wanted him.
Picard knew the borg wanted him - personally - some time before he was abducted.
Since the time he knew, minimal competency required him to take measures to counter this AKA additional security, suicide pill, etc.
And Picard knew he had vital information that must not fall into the hands of the borg.
Picard also knew all other interstellar powers had means of extracting information from an unwilling subject - we even saw some of the cardassian, romulan and klingon technology used for this purpose (it is a LOT better than merely interrogation or torture).
Since the borg were far more advanced than the other powers the federation dealt with, it stands to reason it will have far better ways for extracting information (as it turns out, it does have such means - assimilation).
Minimal competency required Picard to consider the worst case scenario - that the borg do have the means to extract information from him - and implement countermeasures - especially when he had at his disposal several easy to implement countermeasures.
A Klingon and an android seemed security enough.
Thinking a klingon and an android focused on other tasks are 'security enough' against intruders invulnerable to phaser fire and far stronger/faster than a standard soldier is yet another instance highlighting Picard's incompetence.
And if you want to blame Picard for not doing more, neither did Starfleet at Wolf 359. The Saratoga only had the (presumably) Chief of Security on the Bridge. And that was after the Enterprise was able to tell the fleet, "By the way, since the Borg captured Picard they know everything he does and another Captain or an Admiral would really help them out a lot too. Good luck in the battle." Starfleet didn't even think to tell the ships warping to Wolf 359 to load as many civilians as possible on shuttles and leave them behind.
For all we knew, all command level officers on the federation fleet received suicide pills; in any case, after Picard, assimilating other starfleet command officers would yield diminishing returns - Picard gave the borg everything they wanted already.
The Borg also mention death as being irrelevant. Could that mean they could still have taken the knowledge from Picard even after a "suicide pill"? Or would you suggest, that once the Borg beamed onto the bridge, Worf should have vaporized the Captain thereby preventing any possible way for the Borg to capture him and take his knowledge?
Really? Now the borg can assimilate (or extract information) from dead persons - something they were never even close to being shown doing? Utterly unsupported speculation.
And Worf/other bridge personnel killing Picard - when it became obvious they can't prevent Picard's capture - would have saved the lives of thousands. As it was, only a highly improbable fluke saved earth/the federation/trillions, seeing how the borg knew by heart everything the federation threw against it, and countered it accordingly. Worf/the rest were willing to pay a VERY HIGH PRICE in order not to kill Picard.
Could more have been done? Yes. Ram the Borg ship. Initiate a Warp Core Breech. Blow up the damn ship. To keep their hands off Picard and do as much damage as possible. Except no one knew why they wanted him, or about assimilation or what it means to be assimilated.
Ramming the borg ship/initiating warp core breach - and causing more than a moderately-sized crated on the borg cube - would be far harder than setting a course/engage or initiating self-destruct.
In other words, it would have required brilliant planing/genius. One can't fault Picard for not coming up with such.
But Picard failed to even rise to the standards of minimal competency - to take the most basic, straight-forward measures required by the situation. And that IS IMPUTABLE to Picard.
Capturing the Captain to them, at that time, meant no more than if the Romulans had captured him. They would get information, but it would be days or weeks before they could use it.
[...]So where are they being naive?
Treating the borg as if they were merely romulans - when Picard&co knew the difference in abilities - is proof of incompetence/naivete.
BTW, even the romulans gaining Picard's knowledge would translate into a major disadvantage for the federation - AKA a significantly increased probability of thousands of federation citizens dead, colonies massacred, etc. So yes, even in the case of romulans, Picard should take all necessary measures in order not to be captured alive.
And if they couldn't get Picard, Riker would do nicely, or Shelby, or Worf or Geordi or Data or any member of Engineering. That would have given them what they needed to do what they did at Wolf 359.
The protection measures regarding the bridge/Picard should have been implemented in all the sensitive areas of the ship/all command personnel, of course.