Oh, I think there's
plenty of blame to go around.
But yes, ultimately it falls on Bennett. He had done a great job keeping budgets down in the past, and tailoring stories to what they could reasonably accomplish, but couldn't make this one work.
It probably didn't help that Shatner was not the same kind of director that Nimoy was. Nimoy was by all accounts pretty clear with his vision and knew exactly the type of story he wanted to tell, which is why his scripts are a lot more focused, less scattershot, than TFF turned out to be. I suspect Shatner needed someone to help him formulate a coherent plan and turn all the story elements into making that plan work.
Well...
TVH was a fun escapade, but had so many checkbox list items contrived to be fixed within 24 hours.
Really, nobody else banged into the cloaked ship? Or noticed the indentation where the smooshed grass was not swaying when surrounding grass was? The cloak covers visuals but I'm sure any sounds emitting from the ship would still be there...
The Klingon ship manages to make it after the slingshot effect to make it to 20th century Earth, but we see cutaways of the ship starting to fall apart. No worries, when the script calls for it they warp right on back to the 23rd and nary a problem ensues. Oh yeah, Scotty and especially McCoy are chock full of unusually odd comments too. But that helps build up to the events of STVI - TUC.
Sulu wanders in to get a helicopter with absolutely no credentials, uh-huh... there's a huge subplot quickly and fully sidestepped... he wouldn't have begun to have had enough money to rent one... The novelilzation fills in a gap, but the movie should have told it instead. Let's pretend Plexicorp really owned it and really let Sulu used it for free in exchange for the virtual magic beans Scotty typed up in record time on the Macintosh there (and aren't those 20th century people so easy to believe it too), there are still bigger issues with that subplot and the conveniences and should the audience have to rely on a separate publication to do what the main movie needed to?
Chekov throws his phaser in some misguided belief it'll bounce off all his adversaries' foreheads and konk them all out... so it's in the hands of the ship they trespassed on. No worries, the "it must be the wadaiation" line smooths out all those details (except it doesn't). His communicator vanished as well, surely? But as other aliens gave 1950s humans the transistor, and Scotty gave Doctor Hornball there how to make transparent aluminum in another scene... (for all the complaints people lobbed at more recent series, mistakes were made in the past. The only difference is, various showmakers have said audiences nowadays are more sophisticated - suggesting the scripts need to be better than having all the mistakes made back in the day.))
Also, if the commander took the toy ray gun to give it to his kid and, with said phaser no longer near any radioactive sources (and assuming said commander and crew don't die from radiation poisoning a handful of years later)... I'm pretty sure the radiation didn't plotdevice the phaser out of its functionality as they could still use Uhura's communicator (which would regain strength because radiation probably doesn't work like what was claimed anymore than token sci-fi shows using radiation to give people the ability to make big strong spider webs or big muscles when they get angry and so on. But in Startrekland, it otherwise works that way - except they're pretending they're in our 20th century and that's where suspension of disbelief jumps out the window.)
Yes, they get the nuclear energy gizmo back in time but there's no elapsed time suggesting weeks or months went by before the transparent aluminum panels were made. (Yup, returning to this plot point from earlier.

) All in one day for a completely new technology based on aluminum rather than plastic (a completely separate technology and fabrication process), and if Madelaine really was going in for a nooner as office staff generally have protocols rather than waltzing right on into Doctor Nichols' office after opening the door without knocking...
Who's the pompous ass who yelled "YOU POMPOUS ASS!!" at the Klingon Ambassador, as if that would even begin to help tense relations and fix misunderstandings? Couldn't the scriptwriters wait until the crew got to the 20th century?
The movie starts out with 9 violations, but when the crew are in the trial room only 6 are counted. Also, the old trope of "camera in space" to neatly get the footage of
Enterprise blowing up is a hoot (in-universe; as home video was still too new, these recaps are for the benefit of the audience so I don't think too much into those bits. TSFS also does recapping too, because they didn't want Carol Marcus narrating the Genesis Device again, on top of other nitpicks...)
It's otherwise a fantastic start, but if the alien probe is sucking energy from every ship, overlook how everyone still has gravity but if every ship and station are being neutralized, solar sails won't make a lick of difference because Probey McProbekins will just get that much more power. Solar sails are still a neat idea to try either which way to keep people busy, but the top-level issue is that the probe is sucking energy of all sorts... not to mention, we never found out if the solar sails worked (didn't need to, Spock's guess was a really good one too, though when McCoy said with genuine admiration that Kirk would put Spock's guesses over most peoples' facts, who wouldn't engender a smile?

)
Granted, no movie or TV plot is ever bulletproof, but there are always positive compensations in a film handled right. Nimoy also had more free reign than Shatner had, who had to do the same comedy (at the studio's behest, which I've heard bandied about) because STIV raked it in big so therefore comedy will always work better than doing it seriously when it comes to bringing in the audiences, atop a reduced budget. Add in subject matter that, no matter how tamed, couldn't be done on such a small budget...
Either which way, both movies had plenty of script moments that just about took me out of them. And would for many. Nimoy had better luck with a script that held together just that much better despite it all. Even then, I, II, III, and VI are a lot tighter when all is said and done. And VI screwed up with the a couple clocks being off by a few minutes too... that said, both are also enjoyable in the right mood. IV more so, but some of V's scenes are better than IV's, generally when the plot gets serious again.