Just as I was clearing the cobwebs and I was resolved about kubernetes vs swarm I find myself asking the other question. Why bother? If your business or service is small enough then why swarm the system at all? If your business fits on one server then why build out a cluster? Backups not withstanding the more machines you have the more likely there is to be a failure that is going to require resources to repair. But a properly designed system could be deployed and made safe with minimal hardware.
Frankly there is no reason for a cluster of Raspberry Pi computers except maybe a Tru64 so long as there is a NAS or iSCSI. The thing is there is a lot that can go wrong especially the routers and depending on the volume and capacity a failure could be considerable. I recall the hardware we bought in order to build early giftcard systems. A giftcard or credit/debit card transaction takes so little work that these commodity machines would do the job. Frankly I could make an argument for a rack of laptops simply due to their batteries. The system got more complicated and expensive the bigger and more systems we added.
I keep forgetting about a proof of concept
I worked on after leaving an employer. I built a system that could perform 10-20x the transaction volume and cost 1%, yes one percent, of what we spent in production. Other similar systems were even less expensive.