Wow you're right. Proof of SWORD-Saint doesn't work with any actual Swords (lol), it boosts Katanas, Sabers, Twin Swords, and the odd Dagger (Two Kamui). It should at least work with S-Rank Saber/Katana/Twin Swords under the current logic it follows imo then. (And SJS/TJS since it's essentially a big katana with sword animations/hitboxes. <.<)
You mean a very small group of S-Rank weapons with certain specials are OP. The vast majority of them never see any use, ever, and about half of the specials you get to choose from are garbage. I mean people go on and on about the rarity of Psycho Wand, SJS, Heaven Punisher, etc, but if you wanna talk about rare, just how many times have to seen an S-Rank Saber? S-Rank Knuckle? S-Rank Cane? There's probably more Dodo birds alive today than there are S-Rank Canes. ._.