In Rust's case it is that when you say "you have IsA<Device> for all types that implement IsA<PCIDevice>" you are actually saying "you have IsA<Device> for all types, but only if they implement IsA<PCIDevice>".
When you later say "you have IsA<Device> for all types, but only if they implement IsA<I2CDevice>" the two conditions conflict.
It might be possible to avoid this issue with a different implementation but this is the simplest one that works and QEMU's hierarchy is generally shallow. If a better implementation came along, it would be a matter of search and repeat.
(that said, the C approach is also nice for manually accessing fields of distant base classes)