When I need to type a word using the on-screen keyboard, I often mistype it because of the underlining. This is a limited character set, so it is not the standard iOS keyboard. So, just memorizing where each letter is, doesn’t work here. But because of the underlining, it is almost impossible to distinguish between the “i” and the “j”.
I took a screenshot (top left) and removed the underlining (bottom left). Then I blurred both to simulate less than perfect vision. You will see in the top right that it is almost impossible to see which one is the “i” and which one is the “j”. Heck, even the difference between the “o” and the “p” is hard. But despite the blur, it still is very easy to see which one is which in the right bottom part of the picture.
My vision is not bad, but I am over 40, so it isn’t what it used to be. And I have a background in typography, so I know how much it affects the readability when a text is underlined.