What is the success and failure rate of a senior citizens daycare (adult daycare)?

Hospitals here in Ontario tried to run daycare operations a few years ago but eventually ran into staffing problems. Seniors were in varying degrees of manageability (dementia, alzheimers, wheelchairs, walkers, bedridden), so you can see the problems that would arise day to day. Meals had to be arranged for different diets. It was difficult to say the least.

Now the VON (Victoria Order of Nurses) run daycare centres in small towns throughout Ontario and it actually works quite well because they only take seniors who are manageable who need respit care so their caregivers can have a break.

I can't answer your question because in the long run, there can't be a completely successful senior daycare operation and keep it viable for everyone.

I can personally say I have never known any that have gone out of business, but again most have been well planned and well financed.
First if in the US, and medicare is going to be billed, you can expect 3 to 4 months normal in getting paid though any medicare payment.

Beyond that having qualified staff, proper medically approved meal plans.
And remember your kitchen will have to be inspected and approved by the health department. There will have to be proper equipmenet and rooms for the cleaning of the adult. Each state will have their own specific rules though their state Health department.