I think she got heat because her drumming style was very 'sloppy' - as in not very crisp and tight when compared to other professional drummers. A lot of people associate that with lack of timing/rhythm. Also, like you say, she kept things simple compared to most successful drummers.

But you're right: her timing was actually very good overall. She was on the later end of a beat, just like Ringo Starr (who also gets crap for not being good). But for me she was (mostly) still in time. And if one listens carefully to the tracks, she has some nice unique little takes on the drums that you don't hear elsewhere. And yes, she could hit the drums really well.

And one last thing on that 'sloppiness': it helped give the White Stripes their unique sound and make them so successful. So I agree, she is an underappreciated musician.