Ive had the opposite experence.
Every E4OD ive had has shifted firm. Every C6 smooth as butter.
Oddly enough though ive never had a C6 fail. Ive had 2 or 3 E4OD's give issue.
Mine failed, I don't know why, it was only about 5 years old, and I only remember working on it on the side of the interstate. I had no idea the difference in a C4 and a C6, but the wrecking yard, who required an exchange, only had(or said they only had) a C4 available.
The C6 had the lowest 1st gear, you couldn't leave a stop sign wide open and make it through the intersection (no matter how small) before it hit 2nd gear.
At that time I also had no clue it was a one ton, even though it had a E300 Super Van on the back door. I only learned that while trying to get brake shoes that fit.
It also had 16.5 wheels which made it easy to get tires, because they were mostly owned or used in commercial applications, and got theirs changed fairly early in the wear pattern, making it easy to pick up a set of used ones. I drove one tire that I was counting the plies showing, at 7 I knew there was only one to go, and sure enough it finally sounded off, ChuChu Chu Chu going down the road.
It was also factory red with a white top, and at the time I didn't realize it but I was exceptionally lucky in not getting towed, even one 4 day stint outside the New Orleans police station. I was working a temp job while trying to get offshore and got a ticket while trying to get downtown on Friday to get my check, but this cop said I had to follow him to the station and cover the ticket cost before I could leave. Got out of there about an hour before my hearing anyway, it would have been best to have waited because it would have been "time served".
New Orleans was a tough place, but I finally realized the N.O. Fire Department also used factory Red and White on their vans
It got snagged by a scrapper when I was working in Tennessee or I would still have it