In the trekverse, filled with geniuses (Scotty's and LaForge's level of competence qualify them as such), designing - that is to say, being part of a team that designs - a ship that didn't work properly until O'Brien got his hands on it is hardly impressive.
Scotty and Laforge and O'Brien are exceptional individuals. Is there any reason to think they're typical even of Starfleet?
That fact that during in canon and trek lit, there were presented many other persons of the same caliber (many - but not all - members of starfleet).
If Sisko played the cards right, the starfleet task force actually had a chance of stopping the borg without the use of deus ex machinas. He didn't - and the only reason Alonis survived was plot contrivance: the borg cubes 'froze' just after they neutralized the starfleet task force.
What tactics, specifically, would have worked? Keep in mind that Sisko was serving alongside Vaughn, a man who had been in Starfleet longer than Sisko was alive. If there
was an evitable failure of tactics, I'm more inclined to think that it'd be the one Dax identified in
Destiny of Starfleet ending up in unwinnable battles of attrition against superior forces.
We're talking about the Alonis battle - 6 cubes only. Sisko&Vaughn did not have the task of stopping the entire borg armada by themselves.
At one point in the battle, there were 2 starfleet ships (Sisko&Vaughn) and 3 borg cubes.
As for Sisko's tactical mistake:
When the borg cube started firing on the Alonis city, Sisko had 2 choices:
1. Put his ship in the path of the borg weapon.
What are the likely consequences of this decision? Sisko's ship will, most likely, be crippled. The Alonis city will, temporarily, survive.
Even if Sisko manages to destroy/disable the attacking cube (he did), Vaugh alone cannot win agaist 2 cubes.
The city Sisko tried to protect will survive for all of ~10 minutes before being destroyed along with the rest of Alonis.
2. Do not put his ship in the path of the borg weapon.
What are the likely consequences of this decision? Sisko will, very likely, destroy the borg cube without his ship suffering major damage. The Alonis city will be destroyed.
Sisko&Vaughn only have to deal with 2 borg cubes; these borg cubes can be destroyed by 2 kamikaze attacks on their part (as Vaughn proved).
Alonis will survive (minus one city).
Sisko prioritized saving a city for all of ~10 minutes to saving the rest of an entire world - Alonis. Which is a HUGE strategical mistake.
As it is, the only reason Alonis was not destroyed was plot-contrivance - the author chose not to let Sisko fail to such a degree by using a deus ex machina.
PS - I take it you skimmed RBoE and saw that Sisko's decisions were based only on the DS9 Season 7 warning, with no supplementary knowledge being alluded to or implied, yes?