i would guess that it's hard to get a good (reliable, long-life) seal between the rotating part and the upper surface (i don't think this is much like a wankel

i would guess that it's hard to get a good (reliable, long-life) seal between the rotating part and the upper surface (i don't think this is much like a wankel engine, but it probably does share that problem). also, it looks like it's supposed to spin at a constant, high speed - that might imply high pitched whine (see someone else's comparison with a gas turbine).