It is hard to define that weakness through feats when Oda has ignored far worse damage than the one from stretching while sliced.
However, it is backed by science, logic and manga itself in Enel's arc, through MC admitting it himself.
The way I see Enel's conversation, it was more about slashing being the only means of damaging Luffy for Enel, since he couldn't be pummeled nor electrified. Let's wait until Luffy does fight more blade users eventually. If CP0 is in Wano right now (confirmed by X-Drake), we might see more rankyakus flying his way soon enough.
Now, switching the topic to the top ten...
I like this! Rank your top 10 alliance fighters, go!
1: Luffy, mostly because he'll be the one to beat Kaido (I wish Zoro did because of his connection to Wano and that he uses Enma but realistically speaking I doubt he will defeat any main villain other than Morgan and, eventually, Mihawk). If he masters the top tier CoA, having already mastered Katakuri's level of CoO, he'll be a beast. Not to mention he could also just pull out some G5 out of nowhere right on time for the big boss just like he's been done before with 2, 3 and 4.
2/3/4: Law / Kid / Zoro. Law and Kid are Luffy's main rivals right now as pirate captains and the plot seems to point at them rising to be just below him in the future. I see Law as still a bit stronger than Luffy because of how tricky his fighting style and DF are, but Luffy will undoubtedly surpass both his rivals in this arc. They're also the most likely characters to help him fight Kaido although it will probably end in a 1vs1, as usual. Zoro was shown to be pushed back by some of the scabbards but I don't think we have seen him go serious since the timeskip. I have faith in him performing well enough with his santoryu and Enma to be at least on the level of Law and Kid, if not above them, and getting a bit stronger during the fight (which is what he usually does when he pushes himself in a fight, rather than evolving between them). This arc is the most tied to him as a character and his origins we will ever get until he gets to fight Mihawk so I expect that much from him.
5/6: Inuarashi / Nekomamushi. As
@Aki1991 pointed out, if there's a full moon they're certainly going to become the stronger beasts than the rest of the Scabbards.
7 / 8 / 9: Denjiro / Ashura Doji / Izo. I was extremely impressed by Denjiro and as Scabbards that didn't travel through time both him and Ashura have had 20 years to train and grown stronger. Ashura was also impressive in slashing Jack. Of course that was not enough to take down an ancient zoan (who still hadn't even activated his transformation) but it's impressive nonetheless and I think it's more damage than Inuarashi and Nekomamushi managed to do to him. Izo is here because I expect him to have underwent the same kind of evolution in his skill during these 20 years, despite him not shining in Marineford (to be fair, none of the NW captains other than Whitebeard did, either).
10: Marco. I'm sorry for putting Marco so low. I'd put him together with Law, Kid and Zoro if it wasn't because in this arc the enemies have manufactured kairouseki bullets that even the lowest of underlings use. If Marco tries to use his fruits main power of ignoring damage and instantly regenerating (which is what he's used to doing constrantly) it he could go down in seconds. I'm pretty certain it won't happen as it would be the biggest turn off possible for us readers but I'm just being realistic here.
I'm starting to think I write too much.