Small things that break you suspension of disbelief or immersion

[Frozen_Feet]

Also, different civilizations might disagree on which detect spell is the real detect evil spell (ie, some might mix it up with detect law, for example)

*insert rant of how "good" and "evil" are just labels invented by self-serving gods to stomp on the little guys and totally don't mean what it looks like they mean*

Having gotten that out of the way, "detect evil" is not nearly the useful tool it seems like at first glance, both because it is fallible, and because "being evil" is not equivalent to "being a criminal."

There's also the potential angst of finding out you, yourself, are evil every time it is cast.