I really think I should stop with fillword discussions but here is one more. What happens even if you have set the correct fillword, have made sure all hardware is in tip-top shape and still the encoding errors fly around like a swarm of hornets. Then the problem of ISI might be more problematic.
The main issue still is that the receiving side is unable to distinguish between a 0 and 1. The so called eye-pattern is too narrow or too distorted in such a way the receiver is just seeing gibberish.