Since taking up cycling at age 23, I used toe clips (no cleats) for a couple of decades, followed by a year or two of Power Grips, followed by SPD mountain-style pedals/shoes for a couple more decades. Currently have double-sided SPDs on one of my singles and one-side-SPD/one-side-plain pedals on our tandem and a second single. I very seldom use the plain side on the two bikes with them. Have never used Shimano pedals - have used a variety of Performance/Nashbar/Wellgo/Victor pedals.
Cannot remember ever falling related to foot retention pedals. Of course, I'm working with an older man's memory. If a newbie to clipless thinks that it is inevitable that he/she will fall, that increases the odds that a fall will happen. Self-fulfilling prophecy.
Probably easier to become proficient with clipless if already experienced with toe clips. Important to always be aware of what's going on with the feet/pedals related to what's going on with the ride.
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer