Izuna was around Madara level since they were "equals" when he was wounded by Tobirama.
Slipping up don't mean you are any weaker then you actually are. Everyone make mistakes and his caused him his own life.
MS Izuna (He had around the same amount of training as Madara. Madara was going blind by and he was forced to take his brothers. Meaning that they had trained and fought a lot. Izuna had more training with it then Sasuke.)
MS Izuna > MS Sasuke
Didn't really see too much new stuff since EMS Sasuke but people say he can beat Izuna just because Sasuke has EMS. Having a more powerful sharingan don't mean he is inferior to him though. If Sasuke mess up and try pulling that stuff he did with Madara, he gonna get wrecked.