Material-wise, the bottom plate (to which the drive and electronics are attached) is black plastic, probably ABS. The top surface appears to be a thin, shiny piece of decorative black plastic, glued to a layer of sheet metal underneath for strength. The outside edge is a VERY thick sold block of aluminum--you could probably stand on it. The sheet metal is screwed to the aluminum, with the plastic stuck over the screws, concealing them. It doesn't have a finned piece of aluminum under the drive like the older miniStacks, but the drive does sit on some dense rubber cushions, so it is vibration damped pretty well.
Which is to say the entire top portion is effectively a solid chunk of metal, although it doesn't look like it. Overall the build quality and finish is extremely good, and it's plenty strong enough to support a mini on top of it.
As for wireless interference, never any that I noticed. There does not appear to be any electrical connection between the top case and the electrical ground, so it isn't technically a faraday cage, but I would assume the metal provides some degree of shielding. Besides, it uses an external 12V power brick, so the really noisy power electronics are located elsewhere--everything inside is ICs and DC power.
As for reliability, I had mixed results. The eSATA connection worked fine. The FW800 seemed to work okay, although I had issues chaining anything which may or may not have been the fault of my crusty mini. The USB3 connection, however, had a tendency to drop out periodically, so it wasn't usable as a reliable external.
What I don't know is whether it had the USB problem originally, or if it started flaking out after 1-2 years of use, since I didn't really try to use the USB initially until I started having problems with my Mini and repurposed (or tried to repurpose) the miniStack as a USB3 external.
I can say that whatever controller it uses fixes the spin down problem that the older-style v3 miniStacks had (some--all?--hard drives, when installed in one, would spin down once a minute no matter what you were doing with the drive, which made it largely useless as a primary external).
If you want to buy a used one cheap, I'm about to sell it, since I have no use for it now (I switched to OWC ThunderBays for external storage).