Almost absolutely nothing.
For a start you Amiga will be faster since access to RAM is faster then ROM.
The only slight minus point is that it will take a few more seconds to boot your miggy from cold since it loads the new KS and resets. But this only happens once anyway.
I don't know what support is like for other cards than the Blizzard, but I suppose i ntheory they should be okay as long as they have an MMU...