Hey, just chipping in some thoughts as someone who has dealt with laterally flexy bikes, including one that more or less started to fail in that area--after an absurd number of miles.
I never felt that anything was off when sprinting in terms of flex. Partly because everything moves so much when sprinting, particularly out of the saddle, and partly because it's hard to pay attention to anything other than the road, the riders, and my own lack of fitness when going all out for <20 seconds.
Two places where I noticed things. First, with 2x, I started dropping chains more often, particularly when starting from stops and when climbing out of the saddle (i.e., lower-cadence, higher-torque situations), and the electronic shifting of the FD became less consistent. Second, descending became less predictable. I thought it was the wheels flexing at first, but eventually figured out that the growing softness in the BB was the primary problem. I don't know if this is helpful to folks as they try to figure out how much flex is too much flex between bikes, but I hope it is.
