Many good thoughts here...
Add to them that many grass runways are more casually built on the ground that was found. The result can be runways with unexpected hills, and slopes, which can be laterally across the runway.
For grass runways in parts of the world where winter takes hold, expect a season of thaw, where a runway may look inviting, but be unable to support the aircraft. Private runway owners don't always "X" a runway, which is intended to be closed, and a flyover inspection will not tell you the firmness of the ground. Therefore at certail times of the year, the only way is either a first hand (foot) walking inspection, or a report from a person you really trust. My runway has taken as much as six weeks to firm up, once the snow is gone. Also, the condition in one part of the runway, may not be the same as that in other parts, as has been pointed out.
That said, a smooth grass runway is a delight. One of the many benifits is the fact that the aircraft will slide. This make for much nicer crosswind landings, as touching down with a little crab, does not result in chirping and lurching. This makes grass runways very desirable for tailwheel aircraft. One of the taildraggers I fly, I will land on grass unless there is just no choice.
It is my opinion that if you are regularly making full use of the friction of a dry pavement runway for braking, you are probably using too much brakes. Most GA aircraft into thier "comfortable" runways, should require only a minimum of braking. My grass runway is 2000' long, and I rarely use brakes at all. The less I use them, the longer they last!
Aircraft renters are reluctant about grass runways, just because of the "unknowns". Three of the schools in my area have grass runways which they "approve" for students to use.
With the correct permissions, go look for some grass, it's excellent!
Pilot DAR