I personally also am not a big fan of Incineroar, but still if Ash finally does well with a bulky Pokemon, I'm all for it. He struggled with bulky Pokemon in the past, like Torterra and Goodra, so it's about time he gets a chance to overcome that weakness. Yeah, I know he did well with Snorlax, but Ash's Snorlax was unexpectedly fast for its species.
And regarding detailed design, so was Torterra's and yet Ash had one.
Also I think that depends upon how often Kukui the Masked Royal uses Incineroar, if it's not too often then there is no problem with Ash having one. Paul had a Torterra, and Ash also had one, so there's that. It's just that Paul's Torterra stopped showing up after Ash's Grotle evolved onto Torterra. Same could happen with the Kukui's the Masked Royal's Incineroare.