I tow behind my camper which makes a side door more convenient than climbing over the tongue of the trailer or cutting the truck sharply to the left to deploy the stairs. Some campers also have a lower entry with a side door than on one in the rear if the camper has a basement.
The disadvantage of a side door would be in a tight parking lot and the amount of overhang required to fit a door into the side. If you are looking for smaller camper that can be parked in narrow standard length spots, you're probably looking at model that ends close to the bumper and has no slides.
I've had three trailers before going to rear door truck camper. Although it was convenient to be able to pull forward or back in to a campsite based on the view or topography, I went back to side door for the added access I get when towing. I need extra room to open slides, so I am typically parking in the far end of any parking lot. My truck/camper is now too long to fit one spot without sticking out too much, but I try to choose an end spot and only take up two deep in space. I can deploy my slides over the top of a car that later parks next to me but may only be able to unfold two of the four steps. I have a step stool available for those times or can move the camper if it is too tight. When I am towing, I need 50+ feet of length and typically take up seven spots since I park across them. This leaves me plenty of room for stairs and slides since even compact spots are 16' deep.
Since camping spots typically are set up for trailers, I have not had trouble using a side entry. We have been to RV parks where you camped so close that you cannot open your side awning, but there is still room to open slides and use the side door.