I don't think it's really difficult, it just takes A LOT of practice. There is a lot of failure between knowing a technique, e.g. Bob Ross' sky and cloud blending technique ("two hairs and some air"), and being able to execute it flawlessly every time, with results matching expectations.
In addition, when people start trying, they buy cheap brushes and supplies. I found out the hard way that it makes a big difference. With good supplies things go easier, even for beginners, and one can concentrate on the practice. It doesn't necessarily mean stuff looks good automatically, but it takes away an element of uncertainty. If it ain't the brush or the paint it has to be me, and that's where I can start troubleshooting.
I was never satisfied
Neither am I, not ever. I'm my own worst critic. As an engineer I'm so used to perfection within a given budget; either sh.t works as intended, or it doesn't. Very clear cut - but art doesn't work that way. For me it's difficult to accept that other people might like my productions just as they are and sometimes they even grow on me over time. It's really weird.