I think you're evaluating this as a notation for defining sum types. But that's not what it is. It's a notation for defining a class hierarchy, with all of the additional affordances that gives you.
In practice, most real-world Dart code that uses sealed types also defines methods in those types, getters, and all sorts of useful stuff. Once you factor that in, the data definitions themselves are a relatively small part.
(Of course, you could argue that defining class hierarchies is already intrinscally bad. But Dart is an object-oriented language targeting people that like organizing their code using classes.)
In practice, most real-world Dart code that uses sealed types also defines methods in those types, getters, and all sorts of useful stuff. Once you factor that in, the data definitions themselves are a relatively small part.
(Of course, you could argue that defining class hierarchies is already intrinscally bad. But Dart is an object-oriented language targeting people that like organizing their code using classes.)