For me it’s probably the 7-10 minute pace. I don’t like asynch because of several bad experiences I’ve had where players would put their characters into multiple scenes that would twist up the narrative chronologically and require retcons and other stuff to fix. In one extreme case, a PC died while asynchronously participating in 3 other scenes with wildly separate chronological order to them, and the whole thing left a bad taste in my mouth.
I would say that I’d be okay with asynch or slower if the game had a rule that any given PC could only participate in a single scene at a time.
The whole 0-3 minutes thing seems like it has it’s own pitfalls, but even in my 30 years of playing I can’t honestly remember more than a handful of people attempting it.

