Hey Mav, 12mm on M4/3 equals to 24mm, that's pretty different FOV and it's more propper to compare it with the 24mm on I3.
Honestly the footage I've seen with the 18mm on I3 is very nice. I don't have the 12mm Oly to compare it with the I3. The best comparsion will be with 9mm on M4/3 but I don't have one. I have LAOWA 7.5mm which equals to 15mm on FF and while it gives amazing wide FOV the edges are devastated by tons of blur and vignette. So far the I3 with it's 18mm gives me the better quality but honestly I've had very few ocasions to use it, I'm sure there will be situations in which some cons will be noticable as it is usual for any lens that sacrafices quality for less weight.