Yes, the bump into the armpit also doesn't work too well in my dojo, where many yudansha have judo training, so stepping in that close before you have control of uke creates an opening for a throw. Tani otoshi or sukui nage are the most common responses we get.
If you push down as you "bump", their attempt to throw ends with them face planting themselves into the mat.
However if you don't first have uke's balance before doing this it won't work. Keeping your inside leg back, and stepping forward until uke loses his balance is key. If you have pushed uke forward until he can't keep his legs under him he will go down easily. Any attempts to throw from this position on the part of uke will end badly for him.