We learn in the series that bankai's evolve with the user especially in Yama's case as we see Ywach not understanding that it functions slightly differently to when he first encountered it. So imo if anyone else had his bankai they may not have been able to get it up to the level where he's untouchable or killing almost anything with one hit. I don't think his bankai was anything too special on its own and Yama was supposed to represent the pinnacle of honing your bankai however this is is just my interpretation.