Grav shift is indeed a fragile hold ability with an initial damage which of course causes the interrupt. My guess explicitly would be that it is a bug that they don't "reactivate" after the hold. I do know that engineers used to be able to do the same effect with their covershields and forcefield domes, if you popped it while the workers were shutting down the beacons they would get pushed back, stand up and go back into shutdown mode and be out of range of the beacon, but that has been fixed. As far as other holds, from all of my runs through that is the only hold that stops them dead, they seem to recover from other holds just fine.
Also on a side note, i'm sure that those who have run the cure more then once is aware of the resistances the borg have when fighting them at times. Alot of people for some time speculated that the resistances are a bug, they aren't, but they aren't working totally as was intended, and well, since it came be overcame, the fix is on the back burner. Correct me on this part if I'm wrong, i believe it is just the heavies and elites that get the resist, but maybe normal tacticals as well, been a little while, but they each will have an immunity to 2 damage types at random, for a 30-45 second period. During this time all damage from these types cause no damage at all. They are catergorized as such: Immunities: Beam, Bolt, Physical, Kinetic, Energy, Phaser, Tetryon, Plasma, Polaron, Disruptor, Antiproton. At any 1 time they will have up to 2 resistances active, there is a higher probability of them activating a particular resistance if a specific damage is done. Ie. if everyone is shooting phasers at them higher chance they will immune phaser over disruptor as example. This does not affect the proc's of the weapons, so a plasma fire can still hit them if they are immune to plasma, they just won't take the dot damage until their resistance removes. This was intended to provide the canonical appearance they adapted to your weapons. The big side effect is when they put up resistances to Energy, which blocks all weapons, and kinetic is of course h2h and knockback damage. And when they have both of those up, it's fun to fight for 45 seconds doing absolutely no damage to them. The best bet is to carry a variety of weapons, if you are using all energy, change it up, plasma bolt pistols, disruptor beams as an example, that way if they become immune to bolts or plasma, you can switch to disruptor and beams, since you won't know off the bat if it's the bolts or plasma, if not both, so 2 different types leaves you with a chance that you can hit with the 2nd weapon.
Just some 2cents plus more then what was needed.. sorry i tend to ramble.. :P