Atemi is not "dangerous"
What I would describe as "atemi" as practiced in most dojos is "putting" the strike in place with a relaxed arm. Nobody is hurt by a fisted pressed against their body or held in front of their face (touching their face is just rude). My (less than clear) point is that if my arm is being held utterly immobile in a two hand hold (so as to make any kokyu into a static arm wrestling match), even the biggest guy in my dojo would be powerless to prevent me from changing hamni toward him and putting my fist under his nose. Uke has two choices 1) laugh off an uncontested strike to the face (not bright in a "street" situation) or 2) move. If he moves than it is no longer static and Nage has something to work with. So in the case of Ushiro, I was taught to practice an atemi to the belly as I stepped through under the grabbing hand. Uke is trying to get behind Nage and in the "street" this is a Bad Thing, so why not let the air out of his tires with a belly strike(simulated only) as early as possible. Now most Ukes would start to see a pattern of belly strikes. Uke sould block to the inside and blade their bodies to prevent the strike from landing. This makes the hole for Nage to step through bigger and puts their non-grabbing hand in a very nice to be grabbed by Nage (makes all the hand changing variations of Ushiro Waza possible) If Nage refuses to gently show Uke that he is leaving himself open than his defense is just as dishonest/inept as Uke's attack.