I have wells and septic tanks in most of my rentals in Ocala and Morriston, FL. The wells aren't too much of a problem unless you get iron (I had to put an iron filter at one rental). Also, with wells, ants get into the pressure switches and so no water and tenants go crazy. I keep a couple of extra pressure switches (less than $20 each) because it seems to always happen late nights or on weekends.
As far as septic tanks, the main problem with them is tenants cooking with grease and not being too careful when they dump the grease down the drain. If I have a tenant make me get the tank pumped and it's full of grease, I make sure the pumper shows them the grease and I let them know the next time it happens it's their expense. I use the "home is on a well and septic tank so there's no sewer or water bill" in my ad to rent a house and most tenants prefer not to have the extra bill. I also find that I limit the number of occupants to one more than the number of bedrooms, regardless of age (so normally two adults for one room and one child/baby for each additional bedroom). It cuts down on the amount of water usage on the septic - no matter how old, there are showers, washing machine drainage and dish washing.