I like sandbox mode much better, it seems better for learning if you can just keep going until you are tired.
It seems like there's a bug, I tried changing the settings (different string, different tempo) and with the radio button still set to "sandbox" but it seems like it went into survival anyhow. (I had five hearts and the game ended quickly)
Just fiddling around with it, it seems like you have to let a note ring for a fair amount of time before it figures out what note you are on (just like any tuner I have ever used). I suppose it's just a hard problem to solve, but it seems like that would get in the way of playing the game at a higher tempo or trying to do a similar game that used more than one string at a time (having multiple notes ringing at the same time also seems to inhibit it from zeroing in on what note you just played). Is this why you restricted the game to a single string at a time?
I have not played Guitarsmith, Guitar Hero, or whatever the commercial games are which teach you guitar, but people who use them have told me that they are very accurate with recognizing whether you are playing the note or making a mistake. I'm curious if you have any sense of whether that's true. If so do they just have much more sophisticated algorithms for examining the waveform and extracting the notes?