These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Contested Sequel: GITADORA (2013) due to the removal of the second guitar, making the GuitarFreaks side of the game single-player only.
This is only on new cabinets though. Upgrade kits for XG cabinets exist, and still allow 2 guitar players.
They Copied It, So It Sucks: Initial reaction to the changes to guitar play on XG, because no matter how dissimilar it is (they still don't have hammer-ons and pull-offs, and they did not change to an angled note track either), any guitar game with five buttons is a Guitar Hero rip-off.
And then XG 3 toyed around with having a 3D stage and customizable (but still chibi) avatars. They promptly vanished on the next version.