I completely agree that the switch is genuinely painful to hold on handheld mode for more than ten minutes. The controllers and buttons are too small, it's flat, and generally not ergonomic. It's definitely designed for child sized hands in mind and not adults. I do my best to avoid using it in handheld mode.
Meanwhile, my steam deck is a much superior design. The ergonomics are excellent, it fits my hands, the buttons are spaced apart well, and are adequately sized, and I can play for hours with no hand cramps.
However....
The switch absolutely trounces the deck on portability. The fact that it's flat and small means that, even while in a case, it's extremely easy to slip into a backpack and take on the go. The deck while in a case, however, is bulky and doesn't fit in a backpack if I want to put anything else in there (like a book and my laptop).
They were each designed with different goals in mind. I hate the ergonomics of the switch, but really do appreciate how easy it is to take on the go. I love the ergonomics of the deck, but hate how cumbersome it is to take anywhere. Nintendo made the choice to sacrifice ergonomics, and valve made the choice to sacrifice portability. Unfortunately, no solution will be perfect, and I accept that.