May be a redundant post, but here is everything I've noticed.
Grid2 - No consistency in raid layouts. Constantly have to change the layout from DPS/Tank to Heals for it to fix itself, and that's only temporary. Spacing is always off to the point it overlaps my chat window, overlaps my action bars, or both. Weird spacing when in small groups and pet battles.
Kui Nameplates -
In DPS mode, the nameplates will randomly go grey at low health on targets. Makes it confusing when picking up adds, or which targets belong to our group/raid. I found the option to turn that off.
nibRealUI Config - Whenever clicking the RealUI menu button, a prompt comes up in my chat window that says, "Failed to load nibRealUI_Config: MISSING"
Also, in instances where the extra action button is used (Flamethrowers on Brackenspore, Shell Kick on Tortos), it causes a virtual "dead spot" to occur located in the lower right quadrant of the screen. Clicking the mouse on it does nothing, like moving the camera or dual mouse button click to move, and has the abilities tooltip and description come up if hovered over long enough.
That's all I've noticed. Besides that, still using RealUI after 5 years.