On Google maps streetview I see a sign at the Cedar Breaks junction warning of '8% grades' and '25 mph curves' ahead on UT14 (west bound). My impression from online samples is that Mountain Directory relies mainly on signs like this.
8% for shorter segments is consistent with a long distance average of 6%.
I only see one pair of S curves that are tight enough for 25 mph. That is just above the Cedar Canyon campground.
143 heading downhill toward Parowan has a much tighter pair with 15mph signs (and 'not recommended for trailers' and '13% grade' signs in the ski village). 143 the other direction (toward Panguitch) is a pretty drive. Much of it is high altitude prairie, passing by relatively recent lava flows. I don't recall any significant curves or grades - but I was in a car that wasn't bothered by such.