According to a Gleim study guide,
"Generally speaking, smooth ice on top of the wing is more dangerous than heavy accumulated icing on the leading edge."
Is this true? If this is true, why?
This seems to be from an older Gleim study guide. It references a now-cancelled AC 91-51A:
(AC 91-51A) Ice on the leading edge, while seemingly more dangerous, can be removed using deicing systems. Ice on the upper surface is much more difficult to clear from the structure, thus making it more dangerous.
This AC has been replaced by 91-74B, which repeatedly mentions the dangers of ice on nonprotected surfaces, but also points out that:
Ice on an airfoil can have other effects not depicted in these curves. Even before airfoil stall, there can be changes in the pressure over the airfoil that may affect a control surface at the trailing edge.
and, discussing roll upsets:
If the flaps are extended, do not retract them unless it can be determined that the upper surface of the airfoil is clear of ice. Retracting the flaps will increase the AOA at a given airspeed.
So current guidance seems to be that ice on the upper surface of the wing is dangerous because of:
but it doesn't go so far as to say it is "more" dangerous.