Thanks for posting this! I've been curious about these little keyboards, particularly the A50. I first read about them here several months ago, then a month or two later, Yamaha posted them on their US website, then they (strangely) removed them a few days later. Now, within the past week or two, they are back!
While I do most of my recording with my E433, I could see having an A50 as a sort of "scratch pad" keyboard to take on a trip to get ideas for a song or an arrangement, and it could even be hooked up to Audacity directly if I get a sound I really like. While I'm no fan of mini-keys, they make sense for an instrument like this, and the Reface keybed is definitely a good target when designing a feel for the mini-keys.
A couple things I noticed from your review. You mentioned the headphone volume limit that people with kids would appreciate to help prevent their kids from running headphones at a dangerous level. While this isn't a feature I would need, I would be curious if the that level is maintained if the power is switched off and on, or if the batteries are removed and reinstalled. If not, and a kid figures out they can make the headphones louder by just popping out the batteries and putting them back in, it defeats the purpose.
Secondly, you seemed surprised that, with a C major scale, hitting the "VII" note (B) produced a minor-flat-5 chord. But, that is the chord you get if you play a triad based on the seventh note of a major scale. And yes, it is a soap-opera/horror movie/dissonant chord! For even more fun, add an Ab to the top!