A question I get, usually once or twice a year, is, "would you ever teach anyone to shoot underhand like Rick Barry?" My answer is no.
"But it's so much easier," they say. And I guess that is where I have the biggest problem. In my mind, there is no way that a two-hand underhand shot is simpler than a one-handed shot.
My whole approach to shooting is to eliminate variables as much as possible. The reason the one-handed shot has been so widely accepted is that it does a good job of eliminating the variables caused by the hands. When executed properly you have one hand that does all the work, and another that simply gets out of the way.
Young players tend to get in trouble because they start playing basketball so young with a ball that is too heavy and a hoop that is too high. They resort to an overhead two handed shot just to get the ball up to the rim, but some players never fully break that habit. They end up with a complicated shot and thus are poor shooters. Someone inevitably asks, "what if they shoot it two handed like Rick Barry did? He made 90% of his free throws!"
My biggest issue isn't with the increased variables with two hands. It is the fact that shooting underhand is such a foreign motion to almost every basketball player. Most have never made that motion before and have absolutely no feel for it when they begin. Add to that the increased variables and the fact that there is nothing under the ball to lift it up, or behind it to propel it forward. Put these all together and you've created a perfect storm of potential disaster for players that already fear shooting free throws.
I would much prefer asking a player to relearn the mechanics of a one-handed shot, knowing that the mechanics may carry some positive muscle memory over, and that their shot will be more efficient with fewer variables.
I don't buy into the idea of "desperate times call for desperate measures" although starting over may be necessary.