Well, you can't assimilate what you can't touch, so...

It really depends on if the vessel using it can get it activated before the Borg ship can do enough damage to knock the system offline.

Also, the assumption that it would make a vessel untouchable isn’t necessarily correct. It might be possible to create phased weapons that could hit it, similarly to how the Krenim used temporal weapons.

This is true, but the Borg doesn't seem to have anything like that (otherwise they'd be using them to literally fly past a ship's defenses every time no problem) so they'd have to come up with them first, so it'd still give the cloaked ship the advantage until then.

And even then, the Borg would have to know the precise frequency (or whatever the proper term is) the cloak is phased to in order for such a weapon to work, meaning it'd still require a bit of guessing work, especially if the ship has rigged the cloak to activate on randomized frequencies every time its used.

Of course, the downside to the phased cloak is that every ship using it will have its own weapons out of phase too, meaning it'd probably have to drop the cloak in order to fire anyway. But I think the OP meant more using the cloak to hide and sneak past the Borg than to outright attack them.

It would be effective at first, but the Borg would adapt. They always do.

@Icecreamdif

Yes, but that'd be true of literally any tech you could use against the Borg, but that doesn't mean it isn't still useful to keep using it, especially if you can keep finding ways to at least slow that adaptation. It's what Starfleet did with their shields and phasers, otherwise, by this logic, Starfleet should've stopped using both the moment the Borg adapted to them. Which would be silly, because adapted or not, if it helps ward off the Borg for any second longer than it would without, it's still worth using.

It would be for about 20 seconds before they adapt