Seen it done one or twice, but not a high precentage technique in competition tanto randori due to tantos resistance and counter attack opportunities..
True. From what I've seen and done the trick to getting it off under resistance is to have very solid grounding and an ability to channel your entire body's power through your spine and into your arms to get a good kuzushi. This leaves minimal time to recover from the kuzushi while the rest of the waza is executed. Solid posture is critical. I've also found that if one sets up for Mae Otoshi and it is resisted, Tenkai Kotegaeshi is already half way to completion.
Just some thoughts.