Yeah, it doesn't seem consistent even within the LoR text. I recall that the Uruk-hai were powerful in part because they were unaffected by sunlight. Also, the sun was blotted out for the Battle of Pelennor Fields. OTOH, at other times in lore orcs seem to be doing their thing out in the sunlight without trouble.
I found a Hobbit quote wandering the internet:
"They don't like the sun; it makes their legs wobble and their heads giddy".
-The Hobbit, Chapter 5: "Riddles in the Dark"
It appears this question of orc response to sunlight has come up before amongst fans .