Hi there. I thought I may throw in my two cents, since (cents?) I work at a speedometer shop.
As far as diagnosing why ones not working, as johnceggleston mentioned, if speed and odometer quit at same time, and cruise doesn't work, then there's probably no input from speed sensor. Either sensor bad or a critter chewed the wire (side track- I heard somewhere subaru didn't use the soy-based wiring insulation a lot of manufacturers are so mice and whatnot are less prone to chew on a soob. Any knowledge about this?) anyway, an electric speedo works on frequency (Hz) and some nice multimeters have a function to test this. You would see Hz go up the faster it spins (electric drill is usually fine). No idea the calibration of one of these. It can also be checked on regular dc volt setting, it will show a very low voltage that just jumps around as it spins, so at least you can tell it's doing something. If this checks out good then trace wiring and all connections. As outlined by this repair, I've seen a lot of failures due to simple cold or dirty solders or seemingly benign corrosion on a connection.
I'm glad someone discovered the common problem, I may even get to use it at work. Otherwise we don't really repair any of the modern Japanese electric gauges as there is no info or parts available. Dealer wants to sell you a replacement. But if there's other speedo or gauge issues w any Subarus I'd be glad to look into it anyway, for the sake of the forum.