The article describes the lack of sanitation as a baseless part of the negative mythology, but a dearth of scientific studies (and questions of generalizability of the existing studies) does not mean that their reputation is without merit. When your kid finds turds in the ball pit you don't need a study to know there's a sanitation problem. Anecdotally, as a child I had a near 100% rate of coming down with an infection after visiting the local ball pit and witnessed the discovery of fecal matter by another child, which, combined with the repeated illness, led to a ban on further visits.