Just from a brand perception and vibes perspective and ignoring the actual offerings from DCL, I could see a theoretical "Disney Cruise" offering some amount of luxury, or being rich with ammentities and quality experiences. Disney Princesses, Pirates of the Caribbean, Pixar, Moana, Marvel, Star Wars, Mickey and Co. That feels right to me.
I struggle to find the same positive perceptions towards a Cruise brand with primary themes based around Shrek, Minions, and Jaws. Disney Magic is a real force, and as good as most of the Universal parks are, I don't think it translates 1 to 1 towards a Cruise Ship experience. A lot of big UDX IP are external too. I doubt we'd see Harry Potter or Nintendo here.
Also, from just a basic Google search, it looks like UDX is also currently partnered with Royal Caribbean. The speculation I've heard has been leaning towards a post-2030 launch, so that's not an absolute deal-breaker as there is a lot of time when they could end the partnership before jumping in, but it is something to consider.