Why are some people in the Japanese self learning community so down on Heisig?

You must remember that different communities are strongly biased towards or against different goals, so people who are about efficiency or learning as much as possible before they burn out will totally be down on methods like Heisig, since they add extra steps upfront to the process of getting better at japanese.

I really don't look at it any other way. It's different strokes for different folks. The important part is making sure that people who don't know what they're doing don't get dragged into something that's got caveats attached, and that's a job for time.