When you say flat, I think of comic book like, which is neat and different.
Yeah I never tried painting in that style but he makes it look too easy which means it's probably actually really hard and he's just really good. When I gave painting a go I never had happy accidents either. I probably painting over each canvas I had at least 6 times before I kept anything. I was never satisfied with anything I created.
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.
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.