I have never found harai techniques to be particularly effective. My main experience with both harai men and harai-gote has been to practise them in drill form; and no matter how helpful motodachi tries to be, he knows what is going to happen. Typically he responds with a soft grip that allows his shinai to return quickly to the centre.
Even when I have been successful in this situation, I have felt that it was more the result of motodachi’s kindness than the effectiveness of the harai strike. I faithfully tried to hit the shinai upwards if he has a high kamae or downwards if the kamae is low, but neither of these has given me the breakthrough to make me a dedicated harai convert. Conventional kendo wisdom tells us that harai is likely to be more effective if your opponent is moving forward. This is true, but I still find the outcome to be hit or miss and prefer to try for debana men.
The one opportunity that does seem to work for me is to attack harai-gote as my partner is retreating, either because I am making a strong forward seme, or because I have just failed in another forward attack. Under the circumstances, he is often on his back foot and does not have complete control of the shinai. In this situation, harai-gote is easier to apply than men, as even though I am already in close distance, I am able to strike the shinai at the tsuba end of the jinbu, which has maximum effect in moving the point from centre. Also harai to the ura (kote side) of the shinai has more effect as you are knocking the shinai out of the grip of your opponents right hand; (you are hitting in the direction of his open fingers from the back of his hand). With a harai strike to omote for men, you are pushing the shinai further into his right hand.
So harai-gote has now been added to the keiko tool kit. Harai-men unfortunately looks like it will be a work in progress for another few years