Yes i'm sure, i've all of these devices and Bookari Premium.None of them interpret <u>.
At least epub2, and all of my books/tests are epub2 because the sonys cant handlöe epub3. Maybe eub3 will behave different.
I just mentioned this as a
possible candidate which might leave the Style Buttons. If you have some (more or less) important devices/apps/reasons to keep it, just keep it, i dont care. I myself never saw this element working. Same with margin: auto which should be a replacement for centering, but can be ignored by spec
And no, i will not add a style to my css to format <u> correctly.

If i really really need it i will create a more suitable css rule.