As you can see, RD Tech seems to be currently banned.. and I doubt many (if any) users of it have bothered to use in sunlight, let alone bright sunlight. So, a solid answer might be hard to come.
Note that the UM34C model can apparently be connected to an android app, so, depending on the app's features, one might consider the android device's display sunlight readability instead. Though not many displays are really sunlight readable, let alone bright sunlight readable (I've yet to see one, excluding certain old B/W LCD and epaper types of course), so I wouldn't expect this cheap "LCD" thing to manage well.