People flashing kernels builds over another kernel build without going back to the stock images (e.g. boot image, dtbo image, vendor_boot image, vendor_dlkm image) is definitely one of the problems that does mess people up, unless the kernel developer has said it's ok to flash their kernel over another.
Dirty flashing a factory image (with the -w removed), however, shouldn't be a problem as that method replaces everything with stock, including the kernel (just preserves you data). I've done this almost every month for years with no problems, including on the Pixel 6 Pro with a custom kernel installed. It seems that for whatever reason, however, some people have ran into the corrupt message while doing this, especially on the December build.
Also, some people get a corrupt message but everything still works, and some people can't even boot their phone after getting it.
Dirty flashing a factory image (with the -w removed), however, shouldn't be a problem as that method replaces everything with stock, including the kernel (just preserves you data). I've done this almost every month for years with no problems, including on the Pixel 6 Pro with a custom kernel installed. It seems that for whatever reason, however, some people have ran into the corrupt message while doing this, especially on the December build.
Also, some people get a corrupt message but everything still works, and some people can't even boot their phone after getting it.