Sounds like carbureted engines are more prone to issues than FI possibly? I'm assuming due to floats, bowls, needles, etc.
I'm sure environment plays a significant role, as well; damper and/or colder climates, outdoor storage, etc.
Probably. We've had issues with carbed bikes and regular fuel just due to the heavy detergent packages of some brands.
Also, closed loop EFI systems can tolerate it more than carburettors. However, some warranties are void once the fuel passes a certain %.