One thing I've found trying to find HEPA filters is that many reviews complain of the filters themselves having some kind of stinky off-gassing. I don't know what the reason for this is, or if it's a problem, but it is worrying.

Reviews on most air purifiers seem to have examples of exploding fans and melting components almost burning houses down.

It really seems like there's room in the air purification market for a company that is really trustworthy, effective, and affordable. Apple could do it, for example.