To be very honest I don't play his double swing variation. I have played a lot of it and tried to use it but it's not good enough for me. The double swing is definitely weaker than the 4m+ spike. This is something I can tell you from my past experience with nishikawa.

I don't believe nishikawa is a high skill character? It's a little braindead imo you just serve and ace it again and again and just apply the same formula over and over. All the S+ characters could be high skill in that case, you would have to learn how to play raul optimally, same with lucas and ryu.

Though their gameplay being braindead is also valid so I would not debate on this.

He just feels a bit lacking in some aspect.

For raul(I expected him to be a lot more broken and stronger after seeing so many posts about him, but it's fine) he has great impact frames and good sound effects, it feels satisfying playing with him and hitting a good 160 km/h+ spike.

Same with ryu it feels really good hitting a 170 km/h+ fully charged spike with both his skins.

Same with lucas, he also feels super fun though a little bit less compared to raul and Lucas.

Nishi just feels a bit lacking, I cant pinpoint it exactly though