It's harder. You can get a lot of basic yard work done by yourself, and there are many ways to do marks by one's self, but I believe that nothing beats a real, live, flesh-and-blood person out in the field throwing or shooting.
One of my training groups uses both mechanical throwers, and humans. One thing I have noticed is that dogs who get the majority of their marks from human throwers seem to look OUT more than dogs that get mostly winger-type marks. I don't know why this is.
I train by myself during the week, with two groups on the weekends (one group migrates south in winter), plus one or two evenings during the week, when there's enough daylight.
Lisa