It's one of many examples of how close Poliphony comes to perfection: there is much to like about it, but there is also a catastrophically long list of asinine desing choices that baffles me into oblivion.
The rubberbanding in custom races is insane and there's is no way to disable it: it really kills much of the fun for me, it turns races predictable and you have to go out of your way with ideas to artificially and stupidly handicap yourself in order to create something really resembling a race with unpredictable outcomes.
It's a race where you don't have access to the lap times everyone else is turning, which is probably wise because you would see the insane rubberbanding easily when, all of the sudden, the whole field starts to turn laps 5 seconds faster to catch you, or comically slower to allow you to catch them. I don't remember any sim like racing game where lap times are a forbidden data point.
Would be great if the Custom Race feature allowed you to come up with whole series and supported qualifying. Also, the scaling of time for endurance races is annoying because after 2h you can only set full hours: I would like to make 24h races in 2.4h formats, but that's not possible.
People are incredibly creative with the feature as is. Boggles the mind the lack of a Showcase for it where you could share your events, I have some where I could even provide specific liveries for each car in the field.