They have quality issues with stick drift, and the "single joycon as controller" setup is clearly designed for child-sized hands, but it's definitely an advantage to be able to play the system handheld but also have minimal extra to pack (just the rails widget) if I want to put it on a train table on the kickstand and the use the controllers more ergonomically.

And I mean, if you have kids, being able to double your controllers when they have friends around is also helpful to avoid arguments.