They're also not really tested all that well. They don't always give all the available options you have with some objects and the path finding can be kinda wonky at times, especially when you want to examine something and Manny just starts to walk away from it.
Another potential issue especially for the newer Grim Fandango players and P&C is they may miss some Manny's cues as far as environmental interactions.
With tank controls it's more natural to leisurely walk through any given room or location - at which point Manny's head-turning to highlight interactive objects becomes a lot clearer - whereas with P&C people may be more likely to double-click often and run around rooms and may miss stuff even with the cursor changing on mouse-over.