The power of the ANBU is legendary, but when I say legendary I mean more like a myth. I mean everybody believes its there but who has ever really seen it.
Heck to me it seems that leaving ANBU is a right of passage for those who are actually strong, Itachi, Kakashi, Kisame(There version of it that is), ShiSui, Yamato, the list goes on. Heck even Danzo's subordinates weren't even strong and Danzo seems like the type to surround himself with the best and brightest of the ANBU given that they all obey his everyword. Wait didnt Danzo have Izangi that makes you unkillable for 30 seconds at the cost of 1 Sharingan? Didnt he have like 10 Sharingans along with Shisuis Sharingan that makes whoever you place under a genjutsu follow your every command? And even with that he still couldnt beat Sasuke?