The time travel aspect is so glaringly awful (go back in time before attacking the Federation with just one cube (instead of
twoi) and accomplish the assimilation without any fuss or muss.)
Picard was told they can't bring the ship - the newest of the fleet, which also has a lot of newer and stronger weapons - to the fleet either. Picard cites Starfleet's reason, which then begs questions on everything from season 4 onward... so they get to the battle, nobody tells them to go back, Picard barks an order and everyone instantly jumps and obeys in a massive WTF moment*, and it's clearly only NCC-1701-E that fires the shiny new quantum plotdevices so what the heck was Starfluff doing telling them to stay back or not assigning a shiny new captain a la Jellico?! Not to mention that anyone watching DS9 at the time knows Defiant was created for the sole purpose of attacking the Borg and it's all but destroyed with ease. There's a tradition about ego overriding continuity in Trek, and the tough little ship is no exception when it comes to showing the shiny new NCC-1701-EZ.
The old standby shlock of "The Borg won't fire unless they see us as a threat" surely ceases to apply after their 5th or 20000000th if not 2nd beam-over, surely? The movie is a caricature of the Borg what with that, only sending over one ship and not two if humanity/Federation/the whales breeding in the ocean/etc was important enough to assimilate.
While there were a few plot issues I had found on my own at the time and sadly I no longer have my notepad I brought into the theater I'd taken notes on, Red Letter Media (the Plinkett caricature) has an entertaining review of the film** that addresses just about every every conceivable issue - of which (too) many exist in their exhaustive list - to the point it's the
definitive resource. It's on YouTube and is a surprisingly popular video.
STFC is still
fun, and has a number of good scenes, but I wouldn't take it too seriously. It's a 90s party flick. Like "F*R*I*E*N*D*S" except it's "S*T*A*R T*R*E*K T*N*G"


* So if NEM had the pic of young Picard without hair because they thought the audience was dumb to make the connection if young-Picard had more scalp hair than a hippie, that's nowhere as glaringly awful as what's transpired in the opening/battle scenes of FC alone -- never mind the rest of the movie.
**