Display
I don't believe GSMArena's claim of a maximum brightness of 300cd/m2 to be accurate. In previous reviews they have provided 'Max Auto' measurements for devices that only ramp the brightness up to the true maximum when intense light is sensed by the ambient light sensor. It appears that they have failed to do this with the U11+, or the software release at the time did not offer such functionality.
The panel itself has a very minor matrix-like pattern when looking closely at white pages. As I said above, I'm extremely fussy; this completely blends into the background in normal usage (i.e. for typical usage you cannot see it, such as this page).
The S8 display receives a lot of praise, but decent panel technology is meaningless without decent calibration. Regardless of mode, compared to my calibrated Acer X34 (IPS panel, 94% or so measured sRGB coverage calibrated, though that figure means little by itself) the S8 is extremely yellow and overall completely off the mark. In sRGB mode its rather dull, yet in blue light filter mode certain colours (oranges in particular) almost appear neon-like. By eye at least, the U11 Plus is much closer.
The display has vertical colour shift, towards red. None horizontally. This is less irritating than AMOLED panels such as the S8 (which shifts to a red tint then to blue regardless of angle), but the shift does impact brightness.
Black levels by eye appear to be excellent for an IPS.
Finally, by eye the white point seems closer to 6500K than the iPhone (which is rather cold), and overall colour reproduction appears closer to my calibrated X34 than the iPhone.
All this ought to be verified with a colorimeter; at the very least I'll try to get an accurate maximum brightness measurement as soon as possible. By eye alone, the display may be the best of the bunch.
Form factor
Virtually identical to the iPhone but thicker; expected given the size of the battery. The side bezels are just right, and the display being flat is fantastic; I loathe the S8's curved sides, they make the display feel tiny and in my experience are prone to accidental touches.
Like the design overall, but they're all black slabs (as I like them).
Speakers
All three phones are similar enough in terms of volume. U11 Plus is by far the best; sounds the most natural, with highs not being shrill, merely clear. Using the earpiece as a tweeter sounds very close to a stereo solution, eliminating the need to cup the phone (as is required on the iPhone, and to a lesser extent the S8).
The S8 comes second, sounding thin but at least clear enough for speech. The iPhone is terrible simply, being muffled to the point that understanding people in videos becomes very difficult.