Actually, the Borg started to fail since their usage in TNG as well.
It started with I Borg, Descent, then went on to FC with the introduction of the queen.
Voyager actually kept in line with 'no queen' in Scorpion... but then used her in Dark Frontier.
And most of their 'incidents' with the Collective resulted in Voyager barely getting out alive or always getting help from a force that could stand up to the Borg.
TNG was no different actually.
I agree, except with the Queen.
I think after "I.Borg" & "Decent", you needed to new element added to the Borg to bring them back from wussyville. I didn't mind the Queen in "Dark Frontier" either. After "FC" being one of the highest grossing Trek films, any causal viewer tuning into Voyager would expect to see her. The producers/writers knew that, so for ratings sake they had too. They took her too far in "Unimatrix Zero" because they showed her as weak thus making the Borg look weak again.
If anything, "Unimatrix Zero" should have been a Borg origin story.
Don't show us them as weak, show us what it was that made them strong.
I respectfully disagree on the Queen bit.
It was an unnecessary addon to the Collective which in turn made them look idiotic.
As TNG promptly explained about the Borg: A single leader can make mistakes... in a collective, you minimize the chances of that happening.
I think the collective was rounding up the mistakes ever since the Queen appeared - even though Voyager barely got out of every encounter.
I understand you're issues but this doesnt matter from a production stand point going from small screen TV to a big screen movie. You have to introduce a new bigger element into the films from the TV series or else you asking folks to pay big money for nothing more that a 2 hour TV episode. It was that type of under thinking that made "Insurrection" and "Nemesis" such lackluster.
The same writers/producer from TNG are the same ones on Voyager. Doesn't matter what was quoted in TNG about the Borg, they altered the dynamic of their own creation, just as they've done before with Klingions Trill, Cardassians, etc. We've ignored and often accepted the changes in the past, so we allowed them to continue to make creative changes in the future. We've seen Klingons go from being like pirates to honor bound Samurai. Trek has always been inconsistent in developing alien customs and cultures.