I wanted to install a composting toilet, but I ran out of planning time, and I felt there was just so much stuff I could throw at the local building department at one time. But just like a well, putting in a septic system is no slam dunk. I gather it used to be possible to dig test holes with a post hole digger, put water in them in the driest summer months, record the percolation and you were good to go. No more in Oregon. Test pits are dug and DEQ personnel get down in them and look at the soil strata to find the water table. The strata don't lie. Of the three pits I dug, only one was deemed suitable.
Then one needs enough room for a replacement field. On my 60-acres, I nearly didn't find a place to put my crap. I find this a bit ironic as I could put 30 head of cattle out in the field and no one would blink an eye.
All-in-all, a septic system is an on-site disposal system of sorts, but a lot of ground is disturbed, a 1000-gallon (minimum) concrete tank is buried in your yard and the nastiest sludge has to be hauled off every few years.