Supposedly I heard that players had too many checks per play. Think of it as a majority of the defense having something equivalent to option routes based on what they see. They didn't have definitive assignments basically.
Right if you give everyone checks not everyone will be on the same page. I'm sure it was mostly the secondary with checks, as well as some for the LBs, but you're talking about maybe half the defense possibly not being on the same page and it definitely looked that way at times and you could see guys discussing it between plays.
Whereas when you do it on offense it's between the QB and WR. Even if more than one WR has an option, it's still one read at a time between 2 players.
Where it does get messy on offense is the checks on the line for blitzes and stunts because that's 5-7 guys trying to make that work but those are pre-snap adjustments and not real time like option routes or what Spags does on D.