What Kevin Burwick of MovieWeb says is true regarding dark Rey’s lightsaber,you really don’t get to see it for that long and there’s next to no explanation for it at all unless you happen to read the novel. But the design is still pretty cool and it does possess a few features that are reminiscent of Darth Vader and Darth Maul’s sabers, perhaps for nostalgia or maybe because it’s a little more Sith in appearance. The crackling red blade however is something that one might think she would be better off with since it’s been established that Kylo Ren needed dual exhaust ports on his own lightsaber due to the instability of the crystal powering his blade, meaning he needed somewhere for that added energy to go and thus ended up with a very unlikely cross guard that appears to be just as dangerous for the user as it does for an opponent. Of course if you watch in The Last Jedi during the throne room fight you can see that there are evidently some metals that a lightsaber can’t penetrate or slash through so easily since Kylo does get hemmed in by two of Snokes’ guards, who were presumably handling vibro-weapons, which have been seen to deflect and even hold back a lightsaber in the past.
The whole design of this is pretty simple however since it has a dual purpose if you really look. Not only does Rey gain access to a double-bladed saber with a simple flick of her wrist, but if she decides to do so she can trap a lightsaber between the two blades as well, though this would likely take a lot more practice and an opponent that wasn’t thinking ahead to such a maneuver. Given that it’s fairly new when it comes to lightsabers though it might actually be quite useful. It does sound as though the design team for this unusual weapon had the idea to make dark Rey’s weapon a staff from the start, but that might have been too close to Rey’s light side, which as it’s been shown with other fallen Jedi such as Darth Vader can actually bring them back from the brink. Putting a hinge in between the two halves doubles the threat either way since if it’s folded up it presents a larger weapon that cuts a much bigger swath, while unfolded into a staff it presents the same danger as Darth Maul’s weapon. One major issue with a double-bladed saber however is that it doesn’t have quite the same mobility as a singular saber and does take more energy and more time to master. In a sense, a double-bladed saber is better for dealing with multiple opponents as it allows a fighter to defend on multiple fronts with minimal movement, but it doesn’t do much for damage since it does require much more precise timing and skill to make it worthwhile.
While some might want to put Rey in a higher class of duelist, the truth is that thanks to her inexperience and her still very new adherence to the Force, she’s not nearly a match for anyone other than Kylo, no matter what was seen in The Last Jedi. Luke Skywalker at his prime and in full control of his emotions is still one of the most powerful Jedi ever, and to be honest he’s not even at the top of the list. But a dark side Rey with plenty of training and an ability to tap into powers that she couldn’t possibly fathom would be a formidable foe without any doubt, and the frightening apparition that she witnessed would be something to fear. For Luke it was the fear that he might one day become like Vader, more machine than man, though it’s fair to say that he didn’t fully understand his vision on Dagobah when shown. For Rey, uncertainty was just another part of her life since she’d been left on Jakku when she was very young, but could still remember her parents’ ship flying off without her as she cried for them to come back. The powerful emotions that many Jedi have experienced through the Force and through their lives have helped to shape the Star Wars universe in a big way and given rise to a host of other ideas that have managed to be taken up by the fans and examined through one lens after another. But one thing that’s always bound to be true is that fans will want more, and more, and more back story until there’s nothing left to tell or there’s another story that links to it and is just as enticing.
As far as the question as to whether Rey could take on a darker version of herself, it’s likely she could, but she wouldn’t walk away unscathed.