This is an interesting question.
It's worth noting that, when you buy a doll at retail, that doll has been stringently tested to meet a set of
safety standards. When doll prototypes are made (any toy, really), extras are made with the express purpose of running them through tests to destroy them.
This includes tests for dropping, pulling, breakage, choking hazards, poking hazards, electrical safety, and many other standard tests. If the toy fails, it has to be redesigned. Essentially, these tests are designed to simulate actual play and determine how well the toy will stand up to that play.
Customs do not have to meet these standards, and so their ability to stand up to play will vary depending on the construction of the doll, the materials and the technique used to make the doll.
It is always acceptable to ask the customizer about his or her techniques if you are curious about the workmanship of a doll. Does the customizer hand-stitch or use a machine? What is their technique for rooting hair? What kind of paints are they using, and do they prime beforehand or seal afterward? What materials are being used on the doll?
In my past, I designed items that were both intended for everyday use (Sprite purses), and custom dolls that were very much so 'for display only' (Evil Princess, especially). If a customizer knows ahead of time that you intend to use the doll for more than display, there are substitutions and extra work that can be done to make the doll more play-worthy. But, continue to bear in mind that a custom doll is still a one-of-a-kind handcrafted item, and it should be treated as such.