My Mini 90 is well-finished and has held up fine to moderate use. My Wide 300 is cleanly finished too, though it certainly feels cheaper and more fragile than the Mini 90 or the OneStep Plus - but it too has held up OK so far. A failure after three packs is bad luck, not typical for the line. Just based on subjective impressions of fit, finish and feel in use, I'm more concerned about the longevity of my Lomo Square Glass and Lomo Automat Glass (mini format) than I am about the Fuji or Polaroid cameras. On the other hand, those impressions may not be a reliable guide to what's happening to the parts inside - time will tell. But in any case, at the price, I don't think it's realistic to expect the moving parts in any of these cameras to be built for the ages.
Re composition, they're like any other camera with a plain window finder that's not corrected for focus distance or parallax - you use it a bit, learn how it behaves at various distances, and adjust accordingly.