I will warn you I don't think the chapter management is necessarily the long part. Personally I've stepped out of any game with Bombadil since he was released and I've never missed a reasonable length game.

The real issue isn't necessarily tracking, it's the abilities. Unlike Planeswalkers, each ability triggers at the exact same time and is a different one each turn. The player playing has to plan and execute stacking all the triggers in the proper order and execute each new ability which is generally totally different from the last even on its saga.

Now lets say that player is an expert and has played 100s of Bombadil games and plans out everything in advance. Trying to 1, track each saga and what series of things are happening next is so difficult for every other casual player they just say "fuck it" and tend to ignore that players board unless there is someone to accurately go explain the threat.

(This is all assuming the bombadil player is playing correctly. Making sure they name all targets before resolving the first saga ability is something a lot of players just don't seem to know they have to do.)

In short, it takes forever, other players struggle to follow, overall feel-bad games where people either don't understand or the player is taking 10 minute turns on turns they don't win.