If you can meet the minimum traffic numbers, Exponential is an option. Higher quality ads than AdSense, but the fill rate isn't always as high as one might like.
Demanding min traffic and not being able to fill the slots is a red flag.
It's more of an inconvenience, since you can specify a fallback.
US-based, and they've never missed a payment in 16 years. That's pretty good considering how most ad networks come and go these days.