... With Nikon, the overall exposure compensation and flash exposure compensation is cumulative .. to an extent. For example, if you were to dial in +1.0 exp comp and -1.0 flash comp, it would cancel each other – but only for this scenario where the ambient light is low, and your flash is your main source of light.
Where the ambient light levels dominate, and flash is used as fill-flash only, then different algorithms come into play, and you have other factors such as max sync speed and available apertures affecting the scenario as well .. and hence the flash and exposure compensation might affect ambient light exposure differently then.
But with Canon, flash exposure compensation and general exposure compensation aren’t linked, as they are with Nikon. So with Canon, in manual exposure mode, you can only set flash exposure compensation and not overall exposure compensation.
(It is no use asking me how it handles this in any of the auto exposure modes, since I use my cameras nearly exclusively in manual exposure mode. You’re on your own there.)