Okay here are my experiences.
Cash flow can become tricky when boarders can’t pay on time.
Boarders can be a PIA- there are lots of types out there… the know-it-alls, chronic complainers, the unsafe idiots, gossip whores, some who don’t show up to pick hooves or groom for like… months…, or bring in new nonhorsey type strangers to pet the pretty ponies, I could go on
they are somewhat tolerable when you have awesome boarders to counteract.
You always get the PIA annoying horse too. The one who inevitable is determined to break every damn board in the place. Or kicks, or bites, or cannot be turned out with anyone, or good thing you didn’t have a gun on you or you might have an immediate opening 
Employees can either be self sufficient or royal headaches. Know that you will have to search exhaustively to get the good ones and pay well to keep them!
Plus you have to be prepared for emergency situations… colic, kicks, bumps, bruises, gashes, puctures, seizures. Gotta know what to do until the vet shows up or if the boarder decides they don’t need a vet. (see comment on ‘know-it-alls’).
Sometimes you have to babysit boarders to keep barn sanity. “She did what? with the wheelbarrow, leadrope, and pony???”
Of course there are the financial matters…
vehicles, trailers, equipment, neverending quogmire of insurance options, employee benefits, taxes, feed cost and fluctuations, bedding costs, disposal, utility bills.
I mean you can wing it… Not recommended but many barns can and do fly by the seat of their pants. I mean on paper I probably looked like I was winging it but I did have financial back-up (personal funds) and when push came to shove, I did tap into it a couple times over the years. Overall, as long as the books were breaking even I was content to enjoy running the barn.