Spline lets designers create interactive three dimensional material for websites.
And yes, it can make a website look extremely cool.
The question is whether the visual actually helps explain or sell something.
If you manufacture an interesting product, have a design driven brand, or need to show an object from different angles, I can see Spline being genuinely useful.
If you sell accounting services and add a giant spinning chrome object to the homepage for no reason, perhaps reconsider.
Visual effects should support the story. They should not become the story.
Pros
Visually impressive. Interactive. Useful for products that benefit from being explored.
Cons
Easy to overdo. Heavy visual experiences can distract visitors from the point of the page.
My take
Use Spline when the visual experience earns its place.