Phantoms is DEFINITELY something you don't want to use head on. It's slow charge up and limited range makes it something you can't use on a whim.
However, it is actually much more useful used as an aerial punishing tool, if you can call it that:
For people that recover high - mid heights back to the stage, it's definitely dangerous as the killing power that close to the blast zone is most likely going to kill someone at moderately high percentages.
Another way you can use it:
Purposely missing the 1st hit, but aiming to hit with the 2nd one (the one with all the KO power). Many people underestimate the range of the 2nd hit of a fully charged Phantom Slash because the 1st one is so linear, so you can use that to your advantage.
Usually my mindset of edgeguarding with Phantom is, "When will the opponent just at the right distance so that it looks like Phantom Slash is going to miss, but they'll get knicked by the 2nd strike?" That has been my most practical use for it.
Also, against people with linear vertical recoveries like Marth, Captain Falcon, Ganon etc, the wall box that the phantom creates can sometimes be enough to prevent them from snapping to the ledge if they don't recover properly. I've had a handful of times where the phantom wall box prevents them from moving forward, and they plummet to their doom.
You could also use it the @
Nairo way:
Personally I'm a bit interested in exploring a bit of "towards the stage" Phantom Slashing, but I haven't had the guts to do it quite yet haha.
Finally, I've had a little bit of success where when people are coming down from a high place and you retreating jump back in the direction of their momentum whilst charging Phantom, baiting the airdodge and letting Phantom fly as they're suffering from the after lag. This one's a little trickier, but it's something!
Another really important thing for all these methods is making sure you know your opponent doesn't have any jumps left. It will certainly force a better situation for yourself if you know they're all out of jumping options.
As obvious as this sounds, doing any of these Phantom Slash shenanigans is advised extra caution against people with reflectors. I'm uncertain if the actual slash hitboxes are reflect-able or if it's just the actual advancing phantom that needs to be reflected... some testing is in order!
Hopefully this helps!