Maiden mares can sometimes have a harder time because it is the first time all the muscles get stretched out. So they tend to carry their first babies up high and tight to their bodies. Coming through the birth canal can be a tight fit -- and they may need a little help stretching around baby's head -- which you can asisst with so she doesn't tear.
Once they've foaled, the next pregnancies you may see her dropping baby lower, and doing more stretching out as she gets closer. But even a difficult first delivery is no reason not to breed her again.
Some maidens just pop them out with no difficulties -- that's why being very attentive to them and being there when they foal is so important. You just never know how each one will be -- as each one can be very different.
As you've read in the posted threads, even after a bad dystocias, mares can go on to successfully bring healthy babies with no problems to the ground. The key is being there to help if it's needed.