If it's only going to the range for plinking/fun you may not feel the need for back up sights.
If it's going to be a carry pistol or a home defense pistol you shouldn't rely on just the RMR. Back up sights should be on the pistol in case the battery/electronics croak on you at the wrong time.
The back up sights will need to be taller than the stock sights as the RMR will be in the way after it is installed.
The RMR battery will need to be replaced every so often (lots of folks say they do it once a year on their birthday, anniversary, etc.) and you'll need to go back to the range and sight it in since you have to take the RMR off the slide to replace the battery.
You can get an RMR that doesn't use batteries (some people like them, some don't) as they use either a fiber optic (when there's enough ambient light) or tritium (when there isn't enough ambient light) to light up the dot. The life of the tritium is supposed to be 10 to 12 years. Then you'll have to send it back to Trijicon to have the tritium replaced (no idea what that costs).
The Delta Point Pro also uses a battery, but the battery can be replaced without removing the sight from the slide, so no sighting in trip required after battery replacement. The DP Pro also has the option of installing a rear sight on the back of the sight.
Both the RMR and the DP Pro are advertised as water proof. The smaller shield rds isn't waterproof yet. That is supposedly being worked on to make that sight more robust (read that on another forum where dot sights on pistols - mostly Glocks - is a big thing).
We have RMRs (the fiber optic/tritium type) and DP Pros on four pistols. No issues from any of them yet. But we don't go out and toss any of them onto the concrete, ground, mud holes, etc. either.
After years of "fighting" the idea and cost of an RMR installation on a handgun I can tell you it is well worth it. How worth it? After I got the "test" pistol done, a 9MM P09, I sent out my P07 a week or so later, for the same work. After my wife shot it we sent out her CZ 75 Compact 9MM and had a DP Pro put on it. I'm 60. If I were to lose my glasses I could no longer see the front sight to shoot my pistol accurately. However, with the RMR not having on glasses just makes the dot fuzzy. I have no problem putting a fuzzy dot on the target.