Scyther was really neat in DPP UU for one thing. It was a really brutal offensive mon with its great Speed and good power to make use of, and could run both SD and CB really well. Its CB set in particular was really good with a strong STAB U-turn (See: Scizor), and Technician STAB Aerial Ace to round it all up, which had pretty neat overall coverage barring some mons like Registeel, but with U-turn in tandem with Spikes you can easily wear stuff like Registeel down and gain a lot of momentum. Its SD set was also neat, with Bug Bite and Aerial Ace hitting hard after a boost and allowing Scyther to sweep really nicely in a team. It could run Baton Pass to pass the SD boost if it needed which is cool, and with its nice Speed (tying with the prominent Mismagius), outspeeding much of the tier, it could sweep slower teams really well. Its STABs also do work against some relatively common Pokemon like Mesprit, Venusaur, and Leafeon which helps. But yeah this thing was good. If you play XY/ORAS NU, then Scyther's performance in Gen 4 is kinda similar to how it does in 6th Gen NU. Biggest problem was it had a massive Stealth Rock weakness which sucked hard, but it's not like DPP UU had good spinners (ahem Hitmontop, Donphan, and I guess Kabutops)
It was RU in BW (I played BW RU a lot lol), but it was only decent back then. Its SR weakness was really sucky and it was kind of outclassed by Durant and Scolipede as a Bug-type sweeper, both of which could hit harder and were both faster as well. Durant also had a strong set of resistances which helped it a bit more than Scyther (but that special bulk lol), whereas Scolipede wasn't as weak to Rocks and had Megahorn, EQ, and Rock Slide for all that nice coverage. Both also had Baton Pass which kind of hurt what would have been Scyther's other niche (BP Swords Dance). But that was really its smallest problem back then, it had even bigger ones honestly. For most of BW2 RU it could be revenge killed by the prominent Scarf Rotom-C and Emboar afaik and it struggled against common defensive mons like Alomomola and Steelix. It was a garbage Scarf mon in BW because back in 5th Gen RU, most mons that had a Rocks weakness weren't too great as Scarfers. its use was kind of limited as a VoltTurn mon where it could definitely work with other VoltTurn mons of the time like Manectric and Cinccino to a decent extent. Aside from that, its set of weaknesses left it to be kind of a limited use mon. It wasn't terrible though, and there were far worse mons in RU lol (ahem Evire).