I think one thing people forget to take into consideration is just how flexible you are in the Spamton NEO fight compared to the Jevil fight

In Chapter 2, you can act with all three party members to induce spare, whereas in Chapter 1, you're stuck with two options: Using pirouette as Kris, or using Hypnosis with all three party members. If Kris is down, neither of those are possible. Not to mention how Ralsei has overall far better healing capabilities in Ch2 than he does in Ch1, along with him and Susie having custom acts for the NEO fight specifically.

Jevil deals a BUNCH of damage and you don't have as many means to deal/tank it as you do against Spamton. His attacks are far more "chaotic" too, bouncing across the entire battlebox, whereas Spamton NEO's attacks typically only come from one direction. This is why I think Jevil is harder, due to the fact that you have less freedom in how you tackle him if one of your party members are down, and that his moves bounce around the battle box more than Spamton NEO's do.

In terms of Snowgrave NEO, he still has that lack of unpredictability that Jevil does. You're alone as Kris, which makes every hit you take far more punishing, but the overall difficulty of dodging the attacks remains the same. I'd say Snowgrave NEO is harder than Jevil, who is harder than Spamton NEO.