I think it depends on the location as with a lot of things. We don't call tiny biting flies no see ums. We call them biting flies or gnats (even though they aren't). No see ums pretty much are any small biting insect/bug/mite that you can't see that bites or stings.
I've heard no-see-um used for chiggers and sand fleas, and only those two beasties.
As for seeing chiggers, it's very much possible. It's also highly likely any chiggers you DO see are not the chiggers that are going to go about biting humans, though it is a great idea to not help them out by spreading the adult chiggers all over the place, because then they drop the eggs and it's the babies you gotta watch out for.
Adult chiggers just like plants/plant material - they are harmless. If it's a proper chigger and has eight legs, it won't be biting you (they look like large mites/small ticks, so be accurate in your assessment, or just steer clear!). And where you see the large ones, small ones might not be far away. It's the small six-legged babies that love to chew on animals.
Sand fleas, proper, might not be in the U.S.... trying to figure that one out now. We might just have a likelihood of having actual chiggers on/near some beaches, and there are also biting midges/flies that the young ones might dwell on the sandy surface.