I'm the same as isfet, where I had a plain pen tablet, and bought a screen tablet, but went back to mainly working on my pen tablet.
1) I had to look down and lean over for the display tablet, so it made it worse for my posture and back
2) My hand blocked part of the screen all the time and made it harder to work with
3) I was using a surface pro 6 without the key board, and I missed being able to use keyboard shortcuts quickly. I also had a harder and slower time inputting text into my comics because of the lack of keyboard and having to rely on the onscreen keyboard or pen input
4) Probably just me, but I didn't like the noticeable lag between my pen and the cursor on screen (I used a 3rd party pen with my tablet). I could see it. Meanwhile, if there was any lag on my pen tablet, it was less noticeable because I had that hand-screen disconnect.
5) I didn't have issues with hand-screen disconnect in the first place due to personal experience playing piano, which helped my hand eye coordination. Hence, I actually draw FASTER on my own tablet. Nothing beats traditional drawing speed though. I could get 4 thumbnail pages out in minutes!