The problem that I see is once a organization gets large enough and they start taking in very large sums of money there is going to be corruption, I don't care who it is.
Take a look at just about any non profit group and you will see that the vast majority of the money that is given to them is spent on overhead with less than 10% actually going to what you think.
Many years ago a 2nd Amendment group tried to strong arm me into donating more money. They claimed that they had documents that I had signed saying how much I was going to give them and that they were going to hold me to it. After I threatened to go to their states attorney general did they back off. I'm not going to name them but they are a well known group.
But back to the NRA there are a lot that complain that they are doing very little for hunting. Well, hunting isn't about what they are fighting for. I also don't know just what is required for them to step in when anti gun laws are passed. I know that here in Colorado that Rocky Mountain Gun Owners is the group that is taking all the Colorado laws to court and not the NRA, but after I looked into them I decided not to join them.
In the end a person just needs to do their homework on what groups they want to join and donate money to.