My iPad 3 is pretty much "the Reflec Beat machine" and while it does have a timing adjustment option it only moves the moment in time that it uses for judgement. If I delay it to compensate for wireless earbuds then it will be visually delayed too past the judge line. Sure they can compensate for this with more options but I like sane defaults. I have never had to touch this slider because zero is already perfect, and in games with separate audio and visual lag compensation options I'm never confident that I've gotten it accurate. This isn't something you should have to ask the user to do. https://i.imgur.com/qU2TEP5.jpg

I've pretty much given up rhythm gaming on mobile platforms. There's always going to be too much overhead and too many unknown variables.

Only games that I think have ever gotten this right (insofar as lag correction can be "right") is the Rock Band series, which has auto lag compensation via a photosensor and mic on the guitars.

I imagine the best rhythm game experiences will remain on crappy PCs running Windows XP Embedded for a while.