In TNG, there's no indication that Borg have any particular ability to go through forcefields. As you mention, in both "I, Borg" and "Descent", Borg are detained by forcefields just fine. So, to answer the question you've asked, the forcefields work because that's what forcefields do, and there's no reason at this point in the series to believe they might not be effective.
So, next I'll try to answer what looks like your real question, "Why do the Borg in Voyager have the ability to go through forcefields?" As far as I can tell, this is just yet another of the many ways in which the Borg have adapted to technology, but I'll try to derive what information I can.
One aspect to be aware of is that the writers rebooted/retconned the Borg quite a bit with the First Contact movie, where they made the Borg into even more of a "Space Zombies" archetype than they already were. I didn't find any specific reference in the transcript to any attempts to stop the Borg with forcefields (and I'm not up to rewatching it just yet to be sure), there's just general references to being unable to stop the Borg advancing on the ship:
PICARD: No! Seal off deck sixteen. Post security teams at every access point.
And yet the Borg get through this "seal":
DANIELS: It's pretty bad, sir. It looks like they control decks twenty-six up to eleven. But when they took deck eleven they just stopped.
DANIELS: They're on the move again. The Borg just overrun three of our defence checkpoints. They've taken over decks five and six. They've adapted to every modulation on our weapons. It's like we're shooting blanks.
One would assume that forcefields would be part of this "sealing" and "defence checkpoints" somehow, but as we're not exactly told how the Borg got through them, it may or may not have been by walking directly through forcefields.
The episodes of Voyager that feature the Borg are after First Contact. Here's what I found looking through transcripts of the episodes that Memory Alpha lists as having Borg appearances:
In "Scorpion", when they realize that the Borg are in the vicinity:
CHAKOTAY: I want to cut down on the time it takes to seal off the decks.
KIM: In case we take on some uninvited guests?
CHAKOTAY: Exactly. And I want to increase the magnitude of the forcefields.
Which implies to me at least that there's some expectation that Borg could deal with "lower magnitude" forcefields somehow.
In "The Raven", as you mentioned Seven's Borg nature starts reasserting itself and she walks through "security fields":
CHAKOTAY: She's able to cross through the security fields. She's entering the armoury on deck six.
Though, later a forcefield does stop her:
CHAKOTAY: I'm erecting a forcefield. Harry, reroute all available power to deck ten.
(Seven cannot get through the forcefield across the shuttlebay access doors.)
None of this really establishes the abilities or lack thereof of Borg drones in general, as Seven has been mostly de-Borgified at this point.
That brings us to "Hope and Fear", the episode you're asking about where Janeway assumes that Borg technology would allow them to walk right through a forcefield:
JANEWAY: Not the ringing opposition I was hoping for, but I'll take it. A drone could walk through this forcefield like it was thin air. Is there enough Borg technology left in your body to let it adapt?
SEVEN: If I activate the appropriate nanoprobes, I could alter my bio-electric field. However, I would need to adjust my cranial implant.
JANEWAY: Would a micro-filament do the trick?
SEVEN: It might.
JANEWAY: Then let's get you one. Once you get outside, access that control panel and disable the forcefield. Then we'll try to reach the engine room.
SEVEN: And employ the emergency shutdown procedure.
(Janeway opens her comm. badge and takes out an invisible wire.)
SEVEN: Sufficient. You will need to cross-link the third and sixth nodules.
SEVEN: The adaptations are complete.
(Seven steps through the forcefield and goes to a control panel. The computer alerts Arturis on the bridge. The forcefield is lowered.)
So, either in this analysis (that I've spent way too much time on) I've missed the place where it's firmly established that Borg can traverse forcefields just by walking through them, or in fact this is the exposition establishing it as something that the Borg can do.
From that point on, it seems to be an established Borg trait, though there are occasional attempts to erect "level ten forcefields" in order to contain them.