1. The NRA makes a net profit on every campaign they mail out. If they didn't, they wouldn't do it. It's a case of supreme narcissism to believe the NRA cares about your individual donations, or lack there of. When you're dealing in an organization who's members number in the MILLIONS, it's a pure statistical numbers game. Send out X number of mailings, Y% replies, raise $Z.
2. The NRA isn't just the "one BIG NRA", there is the NRA, the NRA-ILA, and the NRA Victory Fund. By law, the lobby and the PAC that can donate to politicians has to be separate, and they have to maintain their own bank accounts and do their own fund raising.
3. The other groups that do a better job than the NRA of "talking tough" also don't contend with getting dirty up to their armpits actually dealing with the reality of the Congress as it stands, and then go on to do their best. I can't prove it, but I strongly suspect a true accounting of what kind of gun control the NRA has headed off by "selling us out" would be a real eye-opener.
In some instances, I do think the NRA plays it too conservatively, such as the attempted sandbagging of the Parker/D.C. case, but even then, the motives are pure.
I've just recently become a board member of a local state grass-roots RKBA/CCW organization, and in our organization's prior incarnation I was a trusted volunteer and donor. I've been witness to some of the more public shenanigans and made privy to many of the private stories of what goes on in our legislature, and I can only imagine what goes on at the Federal level is only worse.
From what I've learned, I now have a great deal of sympathy for the NRA and what they have to work with. Getting a pro-gun bill passed, a bad one killed, or even just watered down, in the simplest of terms, is a process akin to herding cats as it is. Add to that the betrayals by your "supporters", and the sometimes bitter enemies who can be bought off with the strangest things at the oddest times, and it'll make your head spin.
All in all, I think the NRA does the best job it can. And if you don't like it, as a member, you can vote for a board you think will do better. Check which other RKBA org's have open governance like that.
It's a short list.