In general, on-demand print quality is variable because there are 10,000s of small batch print runs. Even when print quality is high 99% of the time, it still means shipping some badly printed books. Anecdotally, my sense is that quality is highly dependent on the specific region where your book is printed, so you might never get a badly printed copy, but many of your readers in another region might frequently receive low quality print. This has been the largest “hidden tax” of self-publishing for me.
In self-publishing you are spread across a variety of print shops (and time - when it is printed) such that consistency suffers.